I suggest that you investigate hanging those lights vertically. You will gain significant efficiencies so you won't need as many lights, yields will be higher, fans will be smaller, and you have the space. If you go that route be sure to get cool tubes that allows removal of the reflector.

Yes, two exhausts. You likely can cool all of them with one duct if you use a 12" main duct (both in and out) with an appropriately sized fan. It pays to do ducting right. A small leak = big odor. Cooling the lights with an independent exhaust will greatly reduce your cooling requirements assuming you size the fan correctly. In fact, the secondary odor exhaust may provide sufficient temperature control (that may or may not be true in the summer). Again get the CFMs right. Add 15% to your final calculation and be sure to get fan speed controllers for both fans. Having the option of pulling fresh air from outside is never a bad thing except if the temps drop too much. You would need some way to control cold air intake likely via a duct damper.