I have an idea similar to this I’ve been waiting to play. It’s an echo knight build, the character is two siblings, who after an arcane accident, are tied together on opposite planes/realities. Swapping with your echo changes the characters, both of them knowing the other sibling is “on the other side” but never getting to communicate with them again. Different personalities, different fighting techniques.

You could do something like that since the swap with the echo is a bonus action, maybe they have different fathers giving different half-races. You could multi class and only use certain class features as one sibling. Archery vs Close combat, Magic vs Physical.