Hi again, 
I've done a little bit more testing and managed to route the Start Trigger signal from the PXI-5401 to the PXI-6070E: 
With a "FGEN Export Signal" VI I exported the "Start Trigger" signal to PXI Trigger Line 0. With a "DAQmx Start Trigger (Digital Edge).vi" I set the source to PXI Trigger Line 1 and it worked. The FGEN task (Master) is started after the DAQmx task (Slave). 
I tried to export the "Sync out" signal to PXI Trigger Line 0  
and then wired it to the "DAQmx Timing (Sample Clock).vi" as source:  
But I get an error (-200284), which says  "Some or all of the samples requested have not yet been acquired.". 
The "FGEN Export Signal.vi" can export on my PXI-5401 the following signals: 
Start Trigger 
Synchronization Pulse (tried it too) 
Sync out 
(and maybe Sample Clock Timebase, but here I get an error (-1074118653 ) at the Export Signal.vi which says "The RTSI or PFI signal/line cannot be connected as specified.") 
Someone got an idea how the synchronization could be accomplished? 
Kind regards, 
Kleisi