Do two VMs on a single Hyper-V host use the NIC when communicating?
They will not use the physical NIC if the two VMs are on the same vSwitch.
SR-IOV basically takes a physical NIC and passes it through to a guest, so in that case, yes, communication would traverse that NIC since it's not part of a vSwitch.
Ideally, I'd like to see references to actual documentation explaining this, as I've searched, and can't find anything one way or the other.
It would be just as fast for you to fire up netmon on the host and watch the traffic in/out of the NIC and see that there's nothing there for intra-vSwitch communication.