Brian
we run multiple GPS units with our Rugged tablet computers with no trouble.
It is not a big deal to change units. you simply change the port number and Baud in GPS preferences. (the pull down menu under the satellite icon). To speed things up you can configure the external Trimble GPS with the same baud rate as the internal GPS. beware if the Baud rate is 4800 as the computer may think the external GPS is a mouse and the cursor will jump all over the screen. If this occurs then you must disable the serial mouse in the device manager.
also you will need some sort of differential service WAAS, OMNISTAR etc on the external Trimble unit other wise the accuracy gain will be marginal if at all.
Hope this helps,
JL