You keep the Bull and you are allowed to use its special ability (play on two stacks instead of one) for the duration of the game. You may even collect more Bulls and thus play on even more stacks on your turn.
P. S.: It would have been sufficient to post this in the rules subforum.
Sorry should have just posted in the rules only.
Some people say game when they mean round so I want to make it clear.
To rephrase, this means all the rounds of the game, I can place multiple stacks? Sounds like a good strategy to get as many Bull cards as possible if not penalized too much.