Good question. If we've flopped a heart flush, at that point the remaining cards/suits are:
13 spades
13 clubs
13 diamonds
8 hearts
------------
47 total cards
Assuming our opponent is equally likely to have any of these cards in his hand (not a good assumption!), then:
Probability of his first card being a heart = 8/47
Probability of second card being a heart = 7/46
Probability he holds a flush = 8/47 * 7/46 = 56/2162 = 0.026
So about 2.6% of the time, our opponent will hold a heart flush also.
Lastly, if you meant what's the probability before a single card is dealt that 2 or more players will flop a flush in the hand, the answer appears to be (based on Googling) around 0.21%.
13 spades
13 clubs
13 diamonds
8 hearts
------------
47 total cards
Assuming our opponent is equally likely to have any of these cards in his hand (not a good assumption!), then:
Probability of his first card being a heart = 8/47
Probability of second card being a heart = 7/46
Probability he holds a flush = 8/47 * 7/46 = 56/2162 = 0.026
So about 2.6% of the time, our opponent will hold a heart flush also.
Lastly, if you meant what's the probability before a single card is dealt that 2 or more players will flop a flush in the hand, the answer appears to be (based on Googling) around 0.21%.