I have my reasons, but I have 2 gallons of fermented brown ale (1 week in primary) that I want to combine with 2 gallons of yet to be fermented wort.
Is there a reason not to do this? Will I need to add fresh yeast?
Reasons for not doing it would depend on what you're hoping to happen.
I don't see any problem with it as long as you want the added wort to ferment out. You shouldn't need to add more yeast.
Why not keg/bottle the one and ferm the other out like you did the first two gallons? I'd drop it right on the old cake if your first beer is in the primary. Why do you want to combine exactly?
You may be better off fermenting them separately, then once each is done mixing them together.