When my premium membership ends, since non premiums can only play in one at a time?

No, you'll still  be able to play all your current tournament games. You won't be able to join any more tournaments though.

Thanks. Can you play more than one match?

If you're talking about team matches then yep, you can play as many as you like. 

I remember when I got a 7-day free premium membership trial - I took advantage of it in as many ways as I could think of  - and one of them was joining about 5 tournaments (all of which I was allowed to stay in past the 7 days).

Yes I did mean team matches. Also how about tourneys you joined but they didn't start till after your trial was through? Could you still play in them?

I'm not sure I did join any that didn't start until after the trial. My instinct would be that you would still be able to play in them though.

Tks.