If you don't use the base, or you lose connection with the base the rover simply reverts to the performance of the standard Here (M8N).
You can connect multiple GPS units to Pixhawk flight controllers, you should use the same type of GPS when using multiple GPS. You have 2 options for how the GPS are used, blended and switch. Blended, as the name suggests uses a mixture of position data giving you a blended position. Switch, uses the GPS [...]