1. two pair is paired boards
so if the board is KKQ74 and you hold 42o you have two pair
I wouldn't expect that hand to be a winner either
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2. you stack off based on your opponent's range
what does HE stack off with? can you beat that or make him fold by shoving?
for example, if you hold AK on a A73r board you can stack off if your SPR is 5 and AQ is in your opponent's range since he's not folding (if he wanted to fold A high boards he wouldn't be in the pot with AQ to begin with)
but if the SPR is 20, you shouldn't stack off because AQ is not in your opponent's stack-off range unless he's a total fish

say you raise and the board is Axxr and you hold AK
if you bet and get c/r you shouldn't 3b shove unless your opponent is a maniac
but say you 3b pf and get the same board, you should play for stacks against most people because if they called a 3b with AQ they're going to play for stacks (especially since it's like 2-3 bets post flop)
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3. Just bet normally like you have top pair or something
you shouldn't slowplay because a fourth flush card kills the action hardcore and could lose you the pot
second reason not to slowplay is that the ace can stick around to peel two streets so you can get value (when you have a nut flush less hands will peel since second nut flush is not comfortable peeling)