By default, this doesn't track with the internal logic of the magic process. You call up a certain amount of power, and you have to control it. If you exceed the control, that doesn't give you more power - the amount you summoned is the amount you summoned. However, what you're suggesting is not expressly forbidden by the rules, and I'm okay with that.
I guess my only real comment is, I would calculate the stress penalties for summoning power as cumulative for the exchange, because he's drawing the mojo in such a short span of time. So, if I have a Conviction at Good (+3), and I summon a 4-shift evocation, and I roll a Fantastic (+6), I take a 2-stress hit.
If I wanted to use those overflow shifts to do a second evocation attack at Fair (+2), I'd charge *another* 2-stress hit for power, because I've channeled a total of 6 shifts in that exchange. I don't think you should get the price break for having a second action, as far as summoning power is concerned - you just don't have to worry about the control.
Make sense?