Clive Smith, 50, of Bulterley, near Crewe, Cheshire, was found guilty on two charges of conspiracy by a 9-1 majority.
Kenneth Malcolm Clews, 40, of Connin, Conwy, was found guilty on one conspiracy count. On a second charge the jury told the judge yesterday there was no prospect of it reaching a verdict.
The jury had been sitting for more than three weeks considering verdicts and sentence on the two and another man found guilty last week is expected on Thursday.
When the trial started last June, four businessmen were charged with putting out a false Stock Exchange prospectus aimed at persuading both large and small investors to put money into Butte Mining plc - owners of gold, silver and zinc workings in Butte, Montana.
The British company had bought the old mines from the the previous American owners who had ceased operations in the 1970s. But the prospectus claimed there were still valuable and viable reserves remaining at Butte.
One of the four said to be involved in the conspiracy was Dr Herbert Roy Bichan, 53, of Colwyn Bay. Last week the jury found him not guilty on both counts. The trial, estimated to have cost about pounds 10m, followed a three-year investigation by the Serious Fraud Office.
As Dr Bichan walked free, John Clarke, 49, of Gerrards Cross, Buckinghamshire, was found guilty on two counts and will be sentenced later this week.
Yesterday Smith, said by the prosecution to have masterminded the swindle, was found guilty of conspiracy between July 1987 and April 1988 concerning the publication of the share prospectus.