I connected my Yamaha e-piano to my Mac and can jam to any software instrument in Logic Pro.
The piano always sends MIDI input to the currently selected track, which is convenient somehow. But as soon as a friend joins me with a guitar, I cannot play as long as he configures his guitar track. How can I tell Logic Pro to bind my MIDI input to a certain track instead of jumping to the currently active one?
You should be able to record-arm the software instrument / midi track (by clicking the R button which becomes red when active) that you're using with your Yamaha keyboard and then switch to and arm the guitar track as well.
This way you can monitor both instruments at the same time by having both armed, while the guitar is the actively selected of the two.
If I'm understanding correctly, when both are armed you should end up with something like this and be able to play together:
EDIT: Just for clarification, in the screenshot the Guitar is also armed, however the icon is flashing and I captured the screen when it happened to turn off
