I'm actually brewing that exact kit Saturday!
There was another thread on here about splitting up a 10g brew into two 5g carboys. I'm sure the same will work splitting up a 5g brew. Just fill up the first primary to the ~2.5g mark and then fill second primary. Mix wert well though before pitching.
I may actually substitute MM with a rum. Heard it's a pirate porter recipe worth trying. I like your idea though, I could go half MM and half rum... Interesting
Secondary: Barrel Porter
Bottled: Autumn Amber Ale, Caribou Slobber Brown Ale