just to clarify for admins, the OP might be referring to a situation where both males are penetrating each other at the same time (I'm actually not sure if that's what the tag was supposed to be, but let's assume it is). This is possible through a position that's kind of a backwards 69 (where both partners are facing away from each other and their penis are angled downwards (in relation to their pelvis) so they can mutually penetrate each other
It would be nice if a yaoi expert could find some examples. But anyway I think we should call the tag "mutual fuck" or "mutual penetration" so we can expand this already rare tag to "futanari" and double-ended dildos (when used in a yuri setting). I'm sure a lot of people would find it useful
ok, I've just learned something new. This is actually a fair tag then. If anything it might be better to call it "mutual fuck", at the very least we would be able to apply to more than just man on man, as it could be futanari on futanari or even like a two-sided dildo, and I bet many people would actually like that