I just want to ask, can I join two or more signature campaigns simultaneously?
No you cant because you might be marketing for two different competitors simultaneously which might lead to a conflict of interest Or it is dependent by what the signature manager's requirements?
Technically No... by virtual of not putting the company/owners sig then you not part of the sig,simple as that!You need to have a banner right, but what if the other campaign doesn't require it? Is it possible? Thank you.
NO BANNER, NO SIGNATURE 
when its says signature campaign it is what it says unless the avatar space which can be rented out for other forms of marketing or personal use.