I am actually running my base off this setup. I have 2 harvesters harvesting the same 3x3 planter area, each on a different side. I have an AE level emitter connected checking the levels for oak saplings (since that is what I am planting). I have the level emitter connected to vanilla redstone and to my first MFR harvester, then to a redstone torch (inverter), to the second MFR harvester. When the level of saplings drop below my set point, the leaf harvester turns off due to the redstone signal, and the regular harvester takes over to harvest normally and collect more saplings.
Since I want as many leaf blocks as possible, I am only planting a single oak tree in the middle of the planter 3x3. I am using this with MFR fertilizer. I'm powering the tree farm with a single industrial engine. It's enough to run 3 HP boilers--2 off charcoal and 1 off the ethanol produced by plant balls.
Edit: There is a 10 second delay when the harvester switches back on and before it actually starts doing work. I minimize the switching back and forth (losing harvesting time) by using a slow stone transport pipe to send everything back to my base, even though I have my AE network ran to the tree farm. Since it takes a while for all the items in the pipe to make it to my base, the pipe guarantees I over-shoot the mark for saplings. This causes my tree farm to run much longer than it needs to hit the sapling level. This could also be done with some BC gates or MFR rednet logic. Basically, I'm turning on sapling collection when I drop below 64 saplings and turning it back off when I hit 128ish. Without this sort of window, I'd be switching back and forth constantly and lose much of the advantage I have running the tree farm off fertilizer.