I'm still stuck on trying to figure out who exactly this mythical individual would be, that he or she could teleport in, to make "may have been two in the back". I'm sure the cops would have figured that one out by now, between all of Tweedle-Dum and Tweedle-Dee's post-crime stumbles, if this were more than an armchair hypothetical-possible.
Prelim evidence seems to indicate that TM was over-powered/shot very close to home. Any additional person entering that cab would be after TM was murdered. The free space available, the right rear seat/floor area would have been just a bit of a bloody mess for someone to get comfy on.
I'm thinking if anything happened, TM after being shot, was physically transferred (tarped?) from front to rear seat, and MS went to drive the Yukon in convoy - possibly at the named field fairly close by the Bosma residence.
I'm fairly certain that TB had a gory ride to enjoy alone, with no mysterious dark passenger on the way to the farm to pick up the incinerator.