I remember there was a very short lived attempt by a few artists to try and pair these two up. I know very little on Youmu and even less on Futo so I don't know if there were any similarities between the two that would cause artists to pair them up. Only that they are both silver haired cuties.
Regardless of why I do find them cute together and would have liked to see alot more of this kind of art but sadly this pairing didn't go anywhere and has been mostly forgotten it seems. I guess when it comes to pairing Youmu up her only eternal love is Udonge.
There are other pairings I like that seem to have a little more momentum such as SatoRemi and KoiFlan. An old faveorite of mine has always been Chen and Tewi simply for cute animal ear yuri fun times.