We need to nail down your question a bit more. Are you asking "Given that the flop came 789 and two players saw it, what are the chances that they both flop a straight?" Even this isn't enough - we need ranges for both players. I can take a stab at it by assuming that the raiser has a hand in the top 30%, and the caller has a hand in the top 30-50%:
ProPokerTools Odds Oracle Results (2.16 Professional)
Holdem, Generic syntax
Board - 7s8c9d
PLAYER_1 30%
PLAYER_2 30%-50%
1000000 trials (randomized)
How often do(es)
2 players 5-card hand type is a straight on the flop
0.1486% (1486)