I have two MVPx10 units with firmware version 6.03a. If I upgrade one of them to 6.05, will it be able to communicate with the one running 6.03a temporarily, until I can get to that location to update it?

Yes, you can still call between a 6.03a VoIP and a 6.05 VoIP so you will be able to upgrade them one at a time. Make sure that your channel settings stay the same such as interface type and voice coder.