You didn't ask if anyone's twin's partner ever mistook them for the other twin, like a guy shopping at a store and his twin brother's girlfriend come's up and squeezes his ass.
You asked about cases where one twin pretended to be the other in order to engage in an intimate encounter. Even if the other twin was in on the ruse, it doesn't absolve the situation with respect to the 3rd party (rather, it just doubles the number of people plotting to deceive them).
Even if you're just asking out of simple curiosity, it's fair for people to respond that it's a skeevy scenario to be curious about.