Hi , 
as far as i understand this.
you whant to route the signal via syphon and via fullscreen. 
I would say : No , it´s not possible....
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You use syphon because of mad mapper or other apps that use syphon ....
In this case MD8 is a feeder for the other software. 
If you need more outputs i think you would use advanced output. 
But this output does not work with syphon. 
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So : 
Syphon dualhead2go or triplehead2go and Md8 is the way.
But maybe i just wrong and you could explain  a bit why you need Fullscreen and syphon ? 
Because Syphon and the app you route the signal is then "fullscreen" , or not ? 
i don´t know .... just guessing ....
i hope this helps you... and sorry for the english in the French form....
(i´m german.... 

)
pele2010