yep, can add. and often, if you have the two above, you are getting paid by the guy with one.

the ultimate rule, and this will clear up any future questions, is make the best 5 card hand that you can make. so even if the board is 66662, your hand is quad 6's with a fifth card as a kicker. if the board is 5 of the same suit, if you have one of that suit in your hand that improves the flush, it plays.