Twins

I suppose the Engineer and Twin stories I've been telling may have stretched credulity just a wee bit. It's true though, there are two of them. And they are maniacs.
There was an identical brother,
Who looked very much like the other,
As to which twin was who,
There was hardly a clue,
Sometimes they could fool their own mother.

P.S. I've only patted the wrong rump once.