Dan Worrall recently posted a video on Youtube called Super Separator Trick where he explains, how to use combfiltiring to reduce mutual masking of two track which share the same frequency spectrum.
This tried this already in two songs and it seems to be a really effective way, which does not affect the single tracks to much.
The ‘trick’ is, to use the same really short delay on a parallel bus for both tracks, but flip the phase of one of the parallel busses. This way, the frequencies which are attenuated in the one channel are increased on the other channel and vice versa.
Of course you need exactly the same delay on both busses and this is a lot of hussle, becuase you never know, if the setting on one channel sounds good on the other channel.
However, to make this as easy as possible, you can use the same bus for both channels but flip the phase of one sends. This seems to be possible in reaper, as Dan describes, but not in Ardour - at least not that easy - but nearly that easy 
- Use a plugin to flip the phase on one channel,
- directly after that add the send to the ‘comb filter bus’
- directly after that flip the phase again.
And you are done.
As I did not find a plugin (which absolutely does not say there is no…), wich only flips the phase, I created a LV2 plugin Filip which really does nothing else then phase flipping.
So: Happy Combing.
