I have an idea that when trousers were first invented they may have been two separate pieces

Did not the Ice Man, the ancient man found in the Alps have what we would call trousers? They aren't exactly new! I suspect that many technologies have been discovered and have disappeared many times since we humans have been around.

My dictionary shows an origin from Scots/Gaelic, from Old French/Latin "Trebus," which, if my High School Latin serves me well, suggests a three-piece item. So, the left and right legs are joined to a center section to form a single garmet. Speculation on my part, but it seems reasonable to my little old worn out mind.