Can you use multiple focus items in the casting of a single spell?  For example, if I have a +2 power item and a +2 control item that both apply, can I use both or do I have to pick?  (The little GM in my brain says only one at a time, bucko.)  I'm thinking about an Emissary of Power with Sponsored Magic and very little knowledge of magical topics outside of that.  He'll have all 4 item slots thanks to Sponsored Magic, but he can't cram more than 1 or 2 of them into a single item, and he's only got one element to work with (Hellfire).

Would the same apply to Thaumaturgy, when you have the time to physically manipulate multiple tools?  For example, it would seem like Harry could easily use his Miniature Chicago to augment his Lore when preparing for Divination, but then also have a focus item that would help him channel power for the spell more quickly.