Obviously, when two characters, let's say 2 females, are sharing a double_dildo, they are sharing_sex_toy, or sharing_double_dildo even. That's fine.

But would I tag female_penetrating_female? Would I tag mutual_masturbation? Would I tag mutual_penetration? All of the above?

Where do we draw the line of when masturbation ends, and penetration begins?
You could make an argument that penetration has to imply an organic body part. But that would disqualify strapon_sex from being penetrative sex. And if we imply the use of a feeldoe, we went full loop.

What is the correct course of action here?

I think it depends. If one character appears to be actively putting it into the other *_penetrating would apply.