TESTIMONY: Joined at the hip couples - don't you just hate 'em? Marian Coombes explains why two into one won't go
omething very strange happens to some people when they become part of a couple. It is a phenomenon I find so baffling, so myopic, and so stupid that when it manifests itself I want to scream, put a mirror in front of them and say: "See? Two people, two people."

It is a two-pronged phenomenon. First we have the couple's inability to see themselves as two separate people. Thus they fail to realise that a couple drink and eat for two people; which must be why they turn up for dinner parties with one bottle of wine and split the group bill in restaurants as if they were one entity. You tell one of them a life or death secret and of course you don't mind that they told the other. This is called the "One Person with Two Heads Syndrome".

Next comes a couple's inability to operate autonomously: "Sorry, we can't come to the party because Bob's working late." This is called the "Marsupial Syndrome" and in time these people lose the ability to function on their own at all. They believe that when one of them goes away you are just dying to step in (in fact you've lived your life waiting for just that moment) and be the substitute partner.

For the purpose of this article I shall hang all the crimes I have witnessed from various couples over the years, onto one: Bob and Lucy. Bob and Lucy got together when they were 16 and have been together ever since. Lovely. But no longer was I (or my group of friends) able to see Lucy or have her (or Bob for that matter) round to dinner on her own. But you knew when Bob was away because, suddenly, she became free at weekends. "Oh hi," she'd say, "I wondered if you wanted to come over this weekend." She will then ring round all her friends until someone can go and babysit her, because she can't possibly be left alone, in that big house (two bedroomed flat). It doesn't matter that there were times when you were desparate to see her, but "Bob needed her to be there". Nor does it matter that in fact you have a boyfriend that you want to spend the weekend with. She is part of a "long standing couple" and you should make allowances. Bob was unable to attend my 30th birthday because "Lucy had to go to New York".

After parties (where they would turn up, of course, with one bottle) we would share taxis home. They paid as for one, even though others (who shared a flat and therefore were still only going to one address) would pay for two. In restaurants they would proffer their share of the bill and, even though there were 10 of us, the bill was divided into nine. For years, I have to say, I did not even notice this was happening. I was used to giving two presents at Christmas and birthdays and yet only getting one back. But then, one day, something happened that made me very frightened indeed.

One of our friends was getting married and had asked us all to get him a dinner set between us. Fiona, the matriach of our group, organised drinks and we all went round to talk about it and give our donations. At the end of the evening Bob pushed foward their contribution. "That's our bit," he said as he sat back looking smug. We had agreed on pounds 50 each. Each one of us had given that amount (and there were plenty of couples among us). They had given pounds 50 ... between them (even though they both earned a wage). It was then that I realised that they did indeed see each other as One Person With Two Heads.

Lucy and I have fallen out only once. This was due to the fact that she could not understand that when I told her that I was pregnant I did not tell him. And because he has such a big mouth the whole world knew before my sister did.

This isn't a diatribe about how awful couples are. I (like most of my friends) am in a happy, committed relationship. But we go out alone, see each other alone and do not automatically assume that if we are invited somewhere, so are our partners. I hope Bob and Lucy, and couples like them, will be together forever. If they ever split up I have no idea how they will survive - or if they will even want to survive. There is this wonderful saying: "When two people become one the only result can be two halves." I just hope each half picks up their share of the bill.