It says much for Poulenc's perceptive open-mindedness that he so quickly sensed and, to a degree, accepted the necessity of a major shift in French musical culture that he must have realised would prove disadvantageous to his own aims and achievements - even if he had yet to identify the ringleader of the Messiaenistes as an obstreperous 20-year-old called Pierre Boulez. Of course, Messiaen himself had already been around for some time. Born into a provincial family of serious artistic and spiritual endeavour - his mother was a religious poet - he entered the Paris Conservatoire aged only 11, studying with Dukas and duly carrying off all the prizes. But as a rising young composer in the Thirties, he had proved curiously difficult to "place". On the one hand, the systematic rigour of his rhythmic and modal techniques and the surrealistic imagery of the texts he wrote for himself to set seemed Modernist. On the other hand, his religious sentiment and sound seemed to come straight out of what was then regarded as the most conservative native tradition - the French Organ School.
Poulenc, at least, seems to have recognised Messiaen as something of an original, remarking: "Either one loves this music or one hates it, but it cannot be ignored, any more than the paintings of Rouault."
Messiaen, by contrast, must have regarded Poulenc as typical of the smart frivolity of inter-war Paris, against which he had already protested as a member of a short-lived composer group called La Jeune France, preaching a return to sincerity in art. Superbly trained himself, he may also have considered Poulenc a bit of a dilettante. Raised in a wealthy, cultivated haute bourgeoise Parisian family, the latter early took to the piano, but owing to military call-up towards the end of the First World War, he missed out on a Conservatoire training - leaving him with what he acknowledged were technical limitations as a composer.
Unlike Messiaen 10 years later, however, Poulenc found almost instant success in his early twenties mainly because the insouciant, Satie-esque little pieces he came out with so perfectly epitomised the bright, positive, populist new spirit that Cocteau was calling for as a way out of Debussyan Impressionism, and which came to be associated with the loose grouping of composers, Poulenc among them, who were known as Les Six.
Such early typecasting as a musical gadfly had its disadvantages. By the time the French public had begun to realise that Poulenc was also capable of sacred settings of a touching simplicity, closer in spirit to La Jeune France than almost anything by Messiaen, and, moreover, that he had embarked upon a lyrical song recital partnership with the baritone Pierre Bernac ultimately as fruitful as that of Britten and Pears, the Second World War was upon them all.
All the same, to suggest that 1945 was the moment when the musical milieu of Poulenc was finally superseded by that of Messiaen would be simplistic. For a start, Messiaen's religiosity was quite as suspect as Poulenc's in a post-war Paris buzzing with the Existentialism of Sartre. And although he had established himself as a composer of international standing by the time he had completed his vast, kaleidoscopic Turangalila-Symphonie in 1948, Messiaen's real influence was being exerted through his teaching.
A major theme of the analysis class Messiaen ran at the Conservatoire was evidently how the early moderns - Strauss, Schoenberg and Stravinsky - had opened up the most striking new possibilities in rhythm, harmony, timbre and so on in the 1900s, and then had apparently taken fright, retreating into various forms of neo-classicism between the wars, leaving those possibilities still to be explored.
As transmuted by the young Boulez in a series of fierce polemics, this was to emerge as the central doctrine of the post-war avant garde: that henceforth every new work should involve a radical re-invention of the musical language itself. Around 1950, Messiaen himself was galvanised by his star pupil's eloquence into attempting a series of piano and organ pieces of a quite fearsome constructivism. But not for long: soon he was off transcribing the songs of "God's musicians", the birds. And after 1960, his main concern was to amass all of his musical resources - the modes, the birdsong and the constructivism - into the grandly conceived, frieze-like sacred projects of his last three decades.
By then, Poulenc was dead, felled by a sudden heart attack in January 1963 - though from the avant-garde viewpoint he had ceased to matter years before, if he ever had.
Few today would be prepared to argue that Poulenc achieved anything like Messiaen's stature, let alone that, in certain ways, their outputs were complementary.
Yet Poulenc did not simply cease to evolve after 1945. On the contrary, he continued to admire Boulez ("a true musician"), and, in his magpie way, to appropriate clangorous sounds from the New Music - even, in his last choral work, Sept Repons des Tenebres (1962), a patch of Webernian serialism.
Moreover, his three large post-war sacred works plus his opera Les Dialogues des Carmelites (1956) rather strikingly anticipated, as it were by contradiction, the later concerns of Messiaen.
We may deplore on grounds of taste or decorum the outrageous farrago of musical cribs and stylistic clashes that comprise the Gloria (1960) - inspired, Poulenc claimed, by medieval frescoes of angels sticking their tongues out and a vision of monks playing football. But in its evocation of spiritual joy, pain, naughtiness and penitence, the work rather specifically encompasses those "human" aspects of faith that are sedulously excluded from Messiaen's theological visions.
Ultimately, their relative standing may rest upon whether listeners continue to accept the historical view, fathered by Boulez out of Messiaen, that the inter-war years represented a temporary and regrettable surrender to the past in the Modernist programme of perpetual revolution, or whether those years are interpreted rather as the beginning of the post-modernist project of perpetually recomposing the past in terms of the present, as in Stravinsky's collage-like re-animating of traditional forms.
Messiaen's most radical innovation - his alternative concept of form, not as a dynamic process to be worked through, but as a series of durations to be filled in - has undoubtedly had a huge influence in recent decades, not least upon Boulez and Stockhausen.
Yet Poulenc's recurrent impulse throughout his career to push the Stravinskyan approach to its extreme, cramming in unlikely juxtapositions of style and tone as if to test traditional notions of continuity, of "the work" itself, to breaking-point, reveals a conceptual toughness beneath the irony and charm that could yet prove to be quite as salient.
Olivier Messiaen in his Century: Birmingham 90th Birthday celebrations, 10 Dec, CBSO Centre, Birmingham (0121-212 3333), broadcast live on Radio 3; Poulenc Anniversary Series, 12 Dec to 25 Feb, Wigmore Hall, Wigmore Street, London W1 (0171-935 2141)