Never, ever slow play two pair. Two reasons;

1. You should be after BIG pots. Betting two pair will disguise the strength of your hand and get more action from someone with top pair or an over pair. By check calling the flop, check raising the turn (or some other variation) you are telling him I BEAT A PAIR. Bet out that two pair and your opponent may put you on a less strong hand.

2. The obvious...your hand may become the losing hand by giving free cards, or a card may come that will scare away your opponent (ace, paired board, flush draw hits)

BET your hands, BUILD the pot early, and win a huge pot. He either has it or he doesnt, if he cant call you on the flop he probably cant call the run or the river.