This topic is old, but since no one has responded, I guess I will.
No....Logic 6 and earlier can route incoming MIDI only to the one selected Audio Instrument track. The running sequencer can play many Audio Instruments. The only way around this is to have the sequencer running, but then there is that ridiculous delay like with rewire. This all changed with Logic 7 and subsequent OSX versions which can make a very usable host. Under OS9 Cubase 5 makes a great host, though Sound Manager defaults to an annoying 23ms latency. You can get 8ms latency using an Oasys card, for example. Or you can assign Sound Manager itself to that card and get 4ms.