Each sorcery advantage grants you access to two minor favors and one deal. So I believe you can take favors from any deal you know. The fluff is a little more ambiguous over whether you can get deals from multiple dievas or multiple deals from a single dievas. At certain points it actually sounds like being bound to multiple Dievas is possible, but I think its generally accepted that a Sanderis sorcerer can go the entirety of his career with a single dievas. That works better for a role-playing perspective as well. One Devil as a fully fleshed out NPC is a better idea than a whole host of more one note ones. (Though the latter isn't an impossible idea)

The way I read it, the fields represent areas of influence that a Dievas holds.  So if you pull from multiple fields, it shows the Dievas to have more than one sphere of influence...potentially making them more dangerous in the long run.