I don't think either of the signature campaigns operator would want to see two paid signature in one signature space.
MinerSource allows other signatures.
To Join:
1. Put the code below in your profile signature, other signatures are alowed
2. Post on this topic with your BTC addy and your current forum post count
4. Write 50 posts within a month to claim, our signature must be displayed for the full 30 days.
5. If you want to continue for another month, simply make another post 30 days later
6. All participants will also receive a $5 Gift card to our Store, on top of the BTC.
Also I do not see prohibition of other signatures in lightlord's signature campaign
Edit: You might want to verify with him, he may not want anyone else's paid signature at the same space.