I tried the "dirty trick" still in a rehearsal moment and it worked well.
And frankly, it's my only solution, so I'll bet on it. During the process I'll report any problems here.
For anyone who wants to know: my setup is a mac pro dual Xeon "Westmere" with a Radeon 5770. 2 matroxes triplehead2go digital plugged each one in a minidisplay port with adaptors to VGA. The third output of the GPU - a DVI - goes to monitoring.
vídeo file: ABCD
matrox 1: A B C
matrox 2: B C D
Explaining: The single input is a strip video with a 4 screen resolution. I'll send 3 screens to 1 matrox (compounding 1 lateral screen and a wide screen with 2 screens size at middle. To the other matrox I'll send the same wide screen in the middle to double the ansi lumens (at this point madmapper it's absolutely useful since to align 4 ultrashort throws without help it's crazy) and another lateral screen.
How I told before, only with the 2 instances of mapper I could have one output in each matrox and adjust everything with total liberty for each output.
The tests were very good, the performing was ok too, even with audio in the files. But certainly cause it's ONE input and it's a big machine.
Any considerations, except for wishing me lucky ?