If I had several mobile and handheld radios that offer D-Star, I would only register my callsign and use that same callsign for each radio. I would not use multiple initial letter extensions for each radio. By using the same one on each radio, if someone wants to contact me they just use my callsign for the destination. This will route them to the last (connected) repeater I used and then to me. I guess you could make people guess which radio you last used and the unique initial you assigned for each radio, but why do that? You're just goofing yourself and others.
For example, if I visit Colorado I would ID on a local D-Star repeater there so the system knows where I was last at and can route to me. My friends anywhere on the planet would simply use my same callsign as the destination...and it would route to me. Just because one can assign a unique initial for each radio does not mean they should. Still, I am not a D-Star expert and could be wrong.
You can enter N2MWE and two spaces in your radio, or N2MWE A (whichever is currently valid). Then just use that "one" for all of your radios...or not and make people guess and or decide it simply isn't worth bothering.