Best case (If it's a well-engineered chipset/device): One interface is not-connected at the hardware level when using the other, using tri-state buffers.
Probable case: You lock up your computer and external drive, but everything's cool after a reboot.
Worst case: Controller goes haywire and damages the data on the harddrive/damages the harddrive itself. Yes, this is possible.
Bottom line: Don't do it.