If I have two people with PC can I use PC twice during an opponents turn, given I have a clear line of fire with both characters?

Yes, if you have 2 characters with PC, you can PC two times on your oppoents turn, (given you have line of sight to what you want to PC)

In a 'multi-player' game, we've always played that a figure with PC can use it once when it is the controlling player's turn, and once when it is not...and that these opportunities 'reboot' on the controlling player's turn.

So in a 4-player game, player A has PC, players B, C, D do not:

Player A goes first, and can use PC on his turn. If he uses PC on player B's turn, he cannot then use it on player C or player D's turn until after his next turn.

And that is the way its supposed to be. From the PAC:

...Using the same rules, once during an opponent’s turn, this character allows you to force that opponent to reroll one of his or her dice (or die) rolls and to ignore the result of the original roll. In games with three or more players, this power can be used only once between your turns.

Notice it says "an", not "each". If it isn't your turn, you get one, and only one, shot to use any one character's PC.

So you're saying that you have one shot to use PC with any one player during an opponents turn that means if you have say three characters with PC you can only use this power once during an opponents turn

nah, he means that if you have more than two players in the game, you can only use pc from each character that has it once on your turn and once on ONE opposing player's turn.

if you have three pc figs and line of sight in a four player game, you can pc three times on your turn, and three times total among the following three opposing players' turns. once per figure that has pc.

example: i have three destiny figs on the board and just for the sake of argument they have LOS to every character on the map (rooftop positioning or something). During my turn, each destiny can use her pc ability once (three times total). During the next player's turn i can choose to use pc from any of the three destiny figs, but if they use it then, they can't use it again until it comes back around to my turn. so i could use pc from each fig during player B's turn, or I could use it just pc from one destiny fig and save the power on the other two for the consequent players' (C & D) turns.

So, if you are playing with 3 other people. A,B, C, D and you are A.

You can use it during A's turn (attack).

You can use it once, during B, C, or D's turn.

Here's the question: Can you use it if your piece is not involved in the action (and if is every man for himself).

So if B is attacking C, can A make B reroll the dice?

Yes, you just declare which figure is PCing which event. There is no stipulation that one of your figures be involved in the original die roll -- all you need is Line Of Fire to the figure you are attempting to PC.