Just rearised that I have attained the height of a Jr member and very excited about this development that I can do more as a member of this forum. With that being said, I want to know if I can do more than one signature campaign in my account as a junior member.
Congratulations on your achievement, but instead of looking for signature campaigns currently, I suggest you to try getting a good reputation by making well-established posts on which we may give you some merits if you put in some real efforts for the same.
On other note, being a Jr. member doesn't really give you enough space in your signature row, so asking this question currently is of no use.
When you go higher in ranks (like Sr, Hero or Legendary), you will have between 2000 - 4000 characters, so then you may decide whether you wish to put more than one signature in your account (this statement doesn't belong anywhere for signature campaigns).
Though, I've seen some signature campaigns before, those that used to allow their users to put more than one signature at a time even when enrolled in one. If you'll be lucky, you will surely find some campaigns by then whose managers would allow you to put more than one signature and work for both.