Lets say that two models, perhaps acting reactively or in a coordinated order, have both declared BS Attack against my unit, who is only able to see one of them. My unit declares dodge - does this allow him to roll face to face against the enemy he does not see, or does that model that just ignore the dodge?
It will be one Roll against all enemy Attacks (apart from Hacking etc. that can't be Dodged), whether there is Line of Fire or not. From N3 Dodge, my emphasis: In the Active Turn, the Dodge Roll is compared to all attack AROs generated by the active trooper. In the Reactive Turn, the Dodge Roll is compared to all Burst (B) attacks from all active enemies. For example, a reactive trooper could Dodge with a single Roll several shots from enemies using a Coordinated Order or all Close Combat attacks.
Great, thanks. The other question I have then is how modifiers from poor/low visibility zones work in this case - do you add a modifier based on which model you're 'reacting to' (i.e the only model I can see is through a poor vis zone, so there's a -3 to the roll in general) or do you apply it on a per enemy basis (i.e my roll of as 10 counts as a success against against one enemy, but not against another in a low vis zone) - if the second is true, can I ignore low vis zones with dodges against models from out of LoF? (as I have no LoF to them to go through a vis zone)
The MODs will apply separately against each attacker. See the last example http://wiki.infinitythegame.com/en/Dodge Dodging vs a Mine and an Alguacil.