I wouldn't hold my breath in the US waiting for someone to prosecute adult twins
for that.
The general belief seems to be that they are inseparable and indistinguishable from each other and therefore are simply "one"; so finding a jury to convict might be a struggle.
I'm pretty damn sure nobody is going to prosecute conjoined twins for wanking each other.
I mean, if the conjoined twins request separate trials and one twin is acquitted
then who the fuck do they lock up?
I seriously doubt that a US court would even allow one conjoined twin to testify
against the other.
And if one conjoined twin "took the 5th" the other twin would almost certainly
be bound by law to STFU.