With a two drive model, is it possible to have both drives contain the player software and partitions?

Yes. In fact, it is recommended in the Drive Upgrade guide and is part of the normal upgrade procedure.

If it is, if one drive dies on a looong trip, you could always pop out your mini tool set, unplug the other drive, and limp home on half of your music collection?

Correct. I have done precisely this before. Hence the recommendation in the Drive Upgrade Guide.

I even keep a 2.5mm hex wrench in the empeg carrying case.