I guess you could, but it's best to smoke the whole thing. More mass = more moisture, and flats stall enough as it is. Plus, BBQ is as good or even better leftover. I suggest going with an empty foiled pan at higher temps, and tightly foil it once the bark is where you want it. If the flat won't fit though, sure, I guess you can cut in half or do whatever. Just use the foil and higher temps so it won't stall too hard and lose too much moisture before it's tender. I'd add some liquid to the foil of some sort.