I do think they should make the tooltip a little more clear. I think honestly they should just make it so that status effects with a duration overwrite auras and then when they time out the auras would take effect again but currently it seems the auras are all powerful in terms of dominance. This Medusa Head interaction always bugged me at first but it's been there since early access so I've come to accept it as it is.
In terms of usage I tend to get in close to the targets I want petrified with Medusa Head's aura and let two turns of the aura usage play out and then on the third turn I move out of range so the aura is no longer applied and cast Petrifying Visage to get a third turn of petrification since the range on the cast is pretty large it's easily applied at a distance.
