Hi,
You can certainly send the output of any input track to as many Flex Mixes as you want either Pre or Post fader. Flex Mixes can be made into Subgroups so that they can then be routed on to the Main output again but allow you do additional processing before that point.
Alternatively, if you're wanting to do parallel processing you could think about dedicating a second channel on your mixer and using the digital patching to set it to the same input source. You could then use stereo linking to link only the faders of the two channels then process each channel separately.