Thank you all for the input so far! To address a few thoughts:
1) Cost. For this NPC, he already has full blown Evocation and Thaumaturgy (as required for Kemmlerian Necromancy), so I dropped the cost of the sponsored magics to -2 each.
2) IoP Option: I was considering making this an IoP, but after re-reading the explanation of Leyline-Sponsored Magic, and the concept of "bringing the power with you," I thought the idea of personally keeping the box in the city and bringing along with for certain tasks (the box demands souls, so bringing it along to "feed," so to speak, is the extend of bringing it around) was a bit more fitting. The item itself doesn't grant the power; the item is just the vessel that contains the sponsor, so it didn't seem fitting enough to act as an IoP on it's own.
3) Sponsor Agendas: The second sponsor is out to consume souls first, promote the goals of Entropy (think the forces of Down Below) as a close second. In this case, killing targets and/or taking away their free, then using the bodies for necromancy for entropy-like purposes will seem to be the bread and butter of both sponsors.
In this case, would you allow the character to gain a debt to both sponsors as each essentially invokes the same Aspect, or would you require something different? This seemed to be against the RAW (as you can online invoke each Aspect once), but still interesting all the same.
Again, just my notes and thoughts to see if that promotes any further conversations ^_^