Best to leave the Tau Coordinated Fire special rule out of this discussion as it doesn't allow for
combined shooting (I know you know this Verjigorm as you stated this - "and there it's also on a formation by formation basis"):
Quote:
The formations don’t have to stay within a given distance of each other and will complete their actions one after the other.
Quote:
Each formations action is declared and resolved (including testing to see if the target formation is broken) before moving on to the next Tau formation.
I understand where you're coming from there but it's a completely different situation.
The easiest comparison I can think of is a war engine shooting with several different weapons (I know we all know the WE rules - I'm just writing it here for clarity). Whilst it is a single unit, it's weapons are separate (each with own stats). When shooting, they must all target the same formation. This is achieved by rolling all attack dice first and then defender assigning casualties (as per the attackers stated intentions - WE/in or out of cover etc) and rolling armour saves. If the first weapon resolved managed to kill the AV's that some infantry was using for cover, that wouldn't suddenly remove the -1 to hit modifier on the infantry as the dice would already have been rolled.
This is right as it avoids cheesy situations.
It's true that the rules do not cover multiple Overwatches taken at the same time. I always had a nagging thought that it was only ever intended for one formation to be able to use Overwatch at a time. There is nothing to back this up in the rules though.
In the absence of clear rules, my inclination would be to agree with Dave and dptdexys.
Even then it's no clear situation:
Which direction should casualties be taken from (if Overwatchers are firing from different directions).
Crossfire might be available from one Overwatching formation and not another.
Maybe it is intended to allow only one Overwatch per enemy activation. It certainly would simplify the situation!