Two characters deepthroating each other's penises, having the other deepthroat a dildo, or some combination of the two. The post should also be tagged with these tags, because they would apply:

Deep_throat mutual_penetration

Plus:

Fellatio mutual_fellatio mutual_oral mutual_oral_penetration mutual_penile mutual_penile_penetration mutual_penis_in_mouth oral_penetration oral_while_penetrating penis_in_mouth if the characters are deepthroating each other's penises.

dildo_in_mouth mutual_oral_penetration mutual_oral toying_each_other oral_penetration oral_while_penetrating sex_toy_fellatio toying_partner if the characters are making each other deepthroat dildos.

Dildo_in_mouth fellatio mutual_oral mutual_oral_penetration oral_penetration oral_while_penetrating penis_in_mouth sex_toy_fellatio if both are happening, but not simultaneously.