To my knowledge, there's no way to do this as the Time Machine isn't a sophisticated tool, it's just the free software Apple bundled in Leopard. Currently, your Mac should use the ethernet before the wireless, as it knows the ethernet (typically) has more bandwidth available. At any given moment, the hard drive that comes with the Mac will never be able to spin fast enough to utilize the full ethernet connection anyway, so two separate connections is unnecessary unless for redundancy.