Instrument tracks & midi channels

[Frambo]

I'm new to Protools HD and have a basic understanding of midi but cant quite solve the following scenario. (I'm not in front of the Protools computer at the moment so hopefully can describe ok)

The song is using 2 'instrument' tracks, 1 for piano (using Xpand) and the other for drums (also using Xpand). The midi input instruments are a Roland keyboard and a Boss drum machine and the drivers are installed ok. We are not using the keyboard or drum machine as external midi sound sources, only as midi input. The drums were written on drum machine and sync'd to Protools and triggering Xpand or Reason.
The keyboard is on midi channel 1 and the drum machine is on midi channel 10.

We can play the keyboard and access the xpand piano but when we start the drum machine it also triggers the xpand piano as well as the drums. If we click on xpand in the insert box of the drum track the piano sounds stop triggering. If we then go back to play the piano, it triggers the drum sounds as well but when we click on the xpand box in the piano track insert box the drums stop triggering.

However once the midi data has been recorded to a track there is no conflict between drums or piano for playback. At that point we dont need the external midi controllers so we can unplug them and get on with the song but it would be nice to understand how it works.
The conflict only occurs when the external keyboard or drum machine are played. I'm assuming you can use more than 1 midi input at once and I've missed a set up function or can you only use one external midi input at a time despite them being on different channels?

any suggestions and information most welcome. I have looked in the user manual but