Used when a character fingers two of the their holes at once, mainly a character that is fingering both their pussy and anus, though may also apply to a character with a genital slit, if they are fingering their slit and ass at the same time.

It is NOT relevant if the two organs are a pair. For example, a character that has 2 penises using one hand to masturbate each one of their penises at the same time. For this, use double_masturbation

See also:

masturbation
double_masturbation
mutual_masturbation
multi_genitalia
two-handed_handjob