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Basically, if you have two fire prisms, one can "forfeit their chance to fire" in order to supplement the other one.

If I have two fire prisms, Fire Prism A and B, can I do the following:
1.) A Flats out 24".
2.) B Moves combat speed 12".
3.) A "forfeits their chance to fire," and supplements B?

Secondly, can I Flatout with a vehicle, and still fire defensive weapons?

A Fire Prism must voluntarily give up its chance to fire the prism cannon in order to contribute. This means, if for some reason you cannot shoot the prism cannon (flat-out, destroyed, Stunned/Shaken...) that Fire Prism cannot contribute. (Eldar FAQ, 2009)

To answer your second question, no. Moving Flat Out (for Fast vehicles) is akin to moving at cruising speed. You move faster, but you don't get to shoot (or contribute a prism cannon). Also if you move Flat Out you count as obscured and the vehicle will wreck if it gets immobilised on the owning players turn.