I always thought it was because it was 2 flat biscuits sandwiched together with a cream filling? Y'know, like a yoyo is 2 parts with the string between them, just a thought. I remember the mint ones most.
I always wondered why the Banjo biscuit was called Banjo, other than one played in the advert. They were wafer and a peanut filling covered in chocolate and came in a two finger pack, but I digress.....