Hi,
On XP Pro i've gotten messages that say "The USB Port Has Drawn Too Much Current and Will Not Operate Again Until Reset" or something very similar to that. So i suspect that the port gets shut down, but it could vary with the type of motherboard too. MSI for example has detection circuits and i guess others do to, but you might have to find out from the maker of your motherboard.
Another real question is if you try to draw twice the current by using two ports what happens if there is an imbalance and one port tries to deliver more than 900ma. It might shut down. You'd have to be able to ensure that the circuit draws equal power from each port or close to it. That's usually not as simple as connecting them in parallel.