I got too many PMs to reply to everyone, so I'll give a brief outline here and hopefully that will be enough to put it to rest.
There is a lot more background info but these are the basics.
I talked to KG on yahoo and prevail on msn. This used to be different but in the last month or so this was how it was.
A day or so before I found out for sure, my suspicion was raised because the language was starting to become interchangable.
Anyone who has spoken to either of these "two" would realise they HAD quite different ways of speaking. Prevail was all cliched cockney and bravado while KG was quiet and wise and talked like a character out of the Kung Fu TV series.
Over time these personalities began to become blurred. I guess he got sloppy because he felt safe.
About a week ago it occured to me that I'd never actually spoken to both of them at the same time. This alone was a disturbing realisation. Then, out of the blue the next day they were both on line at the same time!
Initially this put my mind at ease. My first thought was "phew, I was wrong". But then as I began to talk to "both" of them, I noticed something. Not once in the hour or two that I was talking to both of them were they typing at the same time. On both msn and yahoo you can see when the other person is typing. This is handy as it stops you interupting the other person or wondering if they are still talking to you or not.
So I started to experiment. I'd type the same question (a loaded one) into both widows and click to submit right after each other, within a split second and waited. I did this several times and each time it was the same. First one would answer then the second. The replies from each of them got shorter and shorter as he tried to keep up.
This basically confimed it for me but I wanted more proof.
On esmb you can see who is online. I noticed only oingo boingo (prevail)was online and that tachikoma (KG) was nowhere to be seen. Someone had recently posted something funny to the board (in the humour section) that I doubted either had seen yet. I looked on the board and noticed that oingo boingo was looking at some other thread (not in the humour section). I pasted the link to the humour section into the window that I was talking to KG in. I then switched back to the board and "TA DA!", Oingo was looking at the thread I had pointed
ONLY KG to. Withing a few minutes he had realised his mistake and quickly logged out of the Oingo Boingo account and into the tachikoma account, but it was too late.
The next day, just as I was wondering how to deal with this "they" came on line again. I played the same game of making him do the "IM dance" and asked if we could conference. There were many excuses given as to why that couldn't happen.
To cut a long story short, I just got pissed off by the bullshit and told him straight out that I knew. I lied and said I wasn't angry at all, just curious as to why he would do this. He then admitted everything and gave me a few excuses as to why he did it. I then told him that I lied about not being pissed off
A lot of swearing then followed and I told him to post to esmb and ocmb tell everyone about this or I would. He accused me of blackmailing him!!! Oh the irony!!

In the end I told him to leave and he did.
He never posted to either board to explain himself, so, as promised, I did. I feel that a lot of people were caught out by this guy and he needed to be exposed.
I hope that answers everyone's questions.