Technically, the answer is -9, as the discount granted by Sponsored Magic requires you to have Evocation and/or Channeling, whereas Sponsored Magic counts as Channeling and Ritual. However, if you look at the logic behind the price discount, as described on YS287, it would certainly make sense (from a refresh perspective) to price it as -7. That is, you are buying Soulfire-flavored Channeling and Ritual for -5, then adding the hellfire area of focus to both Channeling and Ritual for -1 each (total of -2 for Hellfire).
Given the dyametrically opposed nature of the two power sources, however, I would find it very hard to believe that the second power source aquired wouldn't simply "burn out" the other or some such effect. Ie, a practicioner of Angelic magic who made a deal with a Devil would find their Angelic power source removed from their sheet, so to speak. And even if you somehow convinced both the angels and the demons that you really were on their side, no spell should be able to be cast using both power sources. In addition, it would be highly appropriate to use debt compels from one power source to interfere with uses of the other.
