Sorry to say that as per I understand about signature campaign, it is not possible at all to do or to join multiple campaigns.  Every campaign has their rules and most of them has the same strict implementation to use their signature once joined.  With that, it is too obvious that only one bounty signature campaigns you could join for only one signature you could use on your profile.  Anyways, you could always join multiple social media campaigns aside from signature depending on the multi tasking ability you got.