Participating in multi bounties is against bct rules?
where can i find bounty rules topic ?
Bitcointalk does not have any official rules about bounty campaigns.
If you want to join multiple bounty campaigns then the right is yours. However, you have to make sure that you do the work up to the manager's standards, otherwise you may not get paid at the end of the rounds.
The managers are the ones enforcing the rules, not bitcointalk itself.
Just like providing a service, you are expected to do your work up to certain standards. No difference with bounties.