I prefer to check-raise two pair on the flop if there are several other players in the hand, and I am 95% sure one of them will bet. You don't want to check and have it check around. If I am less confident in that opening bet, I'll take the lead with a pot-sized bet and see what happens.

Two pair is a dangerous hand. You can own with it, or you can get owned... it's just hard for your opponents to sniff out, and hard to get away from if you have it. I don't put much stock in two pair hands that aren't top two pair; I want to take the pot down on the flop if at all possible. So I bet and raise aggressively with those hands. Top two I might be content to sit back and be less aggressive at first, especially if they are higher cards (i.e. KQ on a KQ5 board). That doesn't mean checking it around, but maybe I'll bet less than the pot to encourage calls from average players.

With any two pair in a full ring, I don't recommend pushing all-in as soon as possible. Give yourself time to make informed decisions. Let's say a player raises pre-flop 3xBB, and I call with JT suited. The flop comes AJT two-suited. I check, he bets the pot, I check-raise him... and he raises me right back. This is a good point at which to ponder, what's he got? All of these are feasible holdings:

- AK or AQ; he flopped top pair + a gutshot draw on a dangerous board, thinks he's ahead, and wants to take it down right now
- AJ or AT; he's ahead of you and you're basically screwed
- KQ, see above
- pocket tens, pocket jacks, pocket queens, pocket kings, pocket aces; he could have a set and be way ahead with trips, or he could have second pair + a gutshot draw and be pretty fearless with that hand
- He could also be on some combination of a pair with a flush draw, which will make people a bit reckless

So his range of hands is fairly broad, and he just bet and re-raised. IMO you don't want to see a turn card because whatever he's got, if he's behind it could help him move in front, and if you're behind it's not likely to help you no matter what it is. So find out now. Raise him one more time - even though it's getting expensive - without going all-in just yet. See what he does. If he just calls, you may learn that you're ahead, and if the turn card is harmless that would be a good point to get all your money in. If he goes all-in you have a difficult decision to make. Good luck. Personally, I might fold unless I had a read on the player telling me otherwise.