It's fanfic convention, although the *existence* of SOD Twins *is*
canon.  The only demonstrated effect in the manga was to duplicate
a lump on Happousai's head when a drop accidentally fell on him.

Actually the water fell on a crow which created another crow which
was identical to the last one. That goes to show that it probably is a SOD
identical twins although it would be hard to say what it would do when
splashed on someone who was already Jusenkyo cursed.
Until I find better things to write about. Ja ne.