First off - I can see many reasons to join more then one unit...but I am not going to list all the characters that can do stuff....
As for the argument, I understand what you are saying coming but there is something that you are missing and I just don't think that anyone has pointed it out clearly enough to make it understood....I will try...
Troop A - - - - - - - - - -Troop B
XXXXX
IC - - - - - - - - - XXXXX
XXXXX - - - - - - - - - - - XXXXX
A&B are more then 2" away from each other
The first rule
IC and joining units *
PG 48)
-In order to join a unit, an independent character simply has to move so that he is within the 2" coherency distance of a friendly unit at the end of their Movement phase. If the character is within 2" of more than one unit at he end of its Movement phase, the player must declare which unit it is joining. If a character does not intend to (or cannot) join a unit, it must remain more than 2" away from it at the end of the Movement phase. This is to make clear to the opponent if the character has joined a unit or not. Note that after a character joins a unit, that unit may move no further that Movement phase.
Now, let's go back to movement phase
Troop A -Troop B
XXXXX
IC XXXXX
XXXXX - - XXXXX
Now if this setup was reached by:
Troop A Moving: then according to the rule " If the character is within 2" of more than one unit at he end of its Movement phase, the player must declare which unit it is joining" takes effect and since he is within 2" of more than 1 unit at the end of
HIS movement, then he MUST declare which unit. Since declaring which unit he is a part of is a xingular choice, he could only choose to be a part of one or the other.
Troop B Moving: then according to the rule, the
IC CANNOT join troop B yet since Troop A has not moved yet. Even if Troop A moved first, Troop B was not within 2" when TROOP A (inclufing the
IC), finished THEIR move ment - "end of their Movement phase"
There is no way to move two units at the same time and since the rule SPECIFICALLY states 'Their' movement phase, and referencing the
IC, once he finishes moving/not moving is when the choice is made. There is no ambiguity in the wording - since it states 'their movement phase', once a model is done moving, THEIR movement phase is over. If this stated that 'If at the end of YOUR (or THE) movement phase' - then there would be room for argument - but the word 'THEIR' means the model/unit in reference.
Hope that clears everything up....