Speaking as a DM, I would allow this combination to work. The whole point of the feat is that it allows a spellcaster to be versatile, is it not? What I would not allow is a character using this feat to cast a 2nd level cleric spell by sacrificing two 1st level sorcerer slots, unless that character also had a feat or some other mechanical item indicating they knew how change the nature of their spells, like Southern Magician, Alternate Source Spell or the theurgic feats in Dragon 325.
I'm not sure if this helps you, but it is my thoughts on the matter.