Fixed and variable pre amp outputs

What is the difference between a fixed and variable pre amp output and can both be used at same time for 2 separate amps?
Well, the fixed output is just that: Fixed, invariable and unaffected by the volume control. It is most often used to feed a recorder of some sort.

The variable output is the one controlled by the volume control and used to feed the power amp(s) for listening.

You can use either/both at the same time to feed separate amps but the fixed output, obviously, would need the interposition of a volume control.

That about sums that up.... save for this... you might be able to get the maker of the preamp to "un-fix" the 'fixed' or buffered line out and make it variable as well.

usually this process will enable two sets of variable volume outs, instead of the now only one set.

Sometimes this will as well decrease the gain that is normally applied to the variable outs by as much as half... though not always. Ask your preamp maker to fined out for sure..

IF a second set of variable outputs is desired, simply using Y connectors can do the trick.... easier with RCA connections though, than with XLR.