SO at each Initative step you make Pile In moves as normal, the player who controls the models deciding where they pile in.
Each MODEL then directs its attacks against an enemy unit it is in base-to-base contact with. A unit can consist of models who end up fighting different units.
In terms of coherency, I direct you to the rules for coherency, found in the Movement Phase section of the rulebook.
".....So, once a unit has finished moving, the models in it must form an imaginary chain where the distance between one model and the next is no more than 2" horizontally and up to 6" vertically. We call this 'unit coherency'."
So unit coherency is only applied in the MOVEMENT PHASE after moving.
THis is back up in the following paragraph, starting:
"During the course of a game, a un it can get broken up and lose unit coherency, usually because it has taken casualties from incoming fire. If this happens, in their next Movement Phase, the models in the unit must be moved in such a way that they restore unit coherency. etc
So losing coherency in the Assault Phase is FINE.
As a result, individual models from the same unit can Pile In towards different target units.
Whilst on the subject though, can I add a question for the group?
If we have a combat of four units - Enemy 1, Enemy 2, Friendly 1, and Friendly 2.
E1 is in combat with F1.
F1 is in combat with E1 and E2.
E2 is in combat with F1 and F2.
F2 is in combat with E2.
So if each sentence is a unit, it looks like this:
So the question.
If Enemy 2 is destroyed, resulting in:
Is Friendly 2 still involved in the combat seeing as Friendly 1 linked the whole thing? Do they pile in to Enemy 1?