OK, so it's effectively a way to cram more fuel into the tank.
Looking at the code it seems that if a player wanted both the storage benefit and the injector boost it would not be possible. While the player still has the extra fuel, the tank will not drop below 7.0LY - and if the tanks are full then you can't scoop. I'm happy with that (lack of) interaction.
Actually, I don't think that's quite true as it stands.
There is currently a check for the player buying fuel but not for buying holy fuel, so if you skimmed and then bought holy fuel you'd have a tank and a half of double injector bonus fuel...
It's an easy fix. I'll likely treat holy fuel as standard fuel with regards to this oxp (i.e. buying it removes injector bonus) in the next update.
Meanwhile, enjoy the combined bonus while you can