Yes this forum has no rules in bounty campaign but the manager set the rules by promoting one project and if you violate one of the rules i think you will not be paid, that is why participating in multiple bounty is not allowed it is against in managers rules.
If you want additional income you should participate in social media campaign which is different rules in bounty campaign.
I suggest if you already have a campaign just focus promoting the project and helps them because they will paid for your works.