More important question is why do you want to join 2 bounties at the same time? You usually are only allowed to join one at a time, most campaigns at maximum allow a different logo in the profile avatar but in general not allowed to promote more than 1 on this forum.
So my advice is simple since you cannot add 2 any way. Join 1, and if you want to switch to bountyhive, change the website in your profile.
It is not about signature campaign where is possible to join only one at that time.
Probably it is about airdrop in which adding a page name to the profile is one of the tasks to be performed, or it is some other form of the campaign.
Unfortunately, but regardless of the rank of the account, you can add only one website to your profile.
You can enter other website addresses but in other fields - for example ICQ, AIM, MSN, YIM .. and some more, but these addresses will be visible only in your profile.