There's also steve's cart forest cart. It will shear leaves. Reguardless of what option you choose you will have to have two farms. You can't sheer leaves and get sapplings. Therefore your single sheering farm is not sustainable without support.
You can setup both harvesters side by side and setup a redstone trigger to turn on one and turn off the other when you have a set number of sapplings. So when sapplings are high the sheering one is active, when sapplings are low the non sheering one is active. Make sure there's a gap between the numbers.
You don't need 2 farms, just 2 harvesters. And you don't need the complicated redstone, runs just fine the way I described with both harvesters on constantly, I have used that way many times.