Ok, i'll try and explain this as best i can lol,
In the top right corner of PT screen there's a 'Settings' button, go into that, the first section in that box should be a 'Screen Name -My Screen Name' section, click search and select your screen name, this will now be the player who's stats load up on startup, click apply and go back to the main screen 'General' tab.
I don't know if you have to click refresh to bring up the players stats you just selected but click refresh anyways lol, in the box in the top right there should be a bit with your screen name and site, to the left hand side of that box there's a blue underlined 'Edit' link, click that, click add and type in the other username that you have, click add and it'll say something like, are you sure you want to alias this name to the other username, this might take a while if you have a large database, just click ok and it should do it.
I don't know if it's because you have the trial version and theyve taken some features away though, in the player list on the left can you see both usernames at all??
I go back down to aldershot about 2 o'clock on sunday afternoons normally so making that tourny is a bit hard for me tbh, when i'm home for proper leave though i'll be able to make it, next time i'm home for a while is over easter time so we'll sort something out then man.