While it's certainly doable, I don't know that I'd go with a innernet for use with a tarp. The innernets are made to be tall, with a side opening that corresponds to the Mid's opening. Unless you're going to pitch your tarps quite high, I'd think you'd have netting falling down around you.
I agree with Trevor that the DuoMid with innernet would be a fantastic option instead of a tarp and two bivies, especially if one of you carries the mid and the other carries the inner.
David's right as well, if you're really tall, the mid can get a bit cramped.