Jelly, 1 byte
ż
Pardon my inexperience, I'm just getting started with Jelly. What I believe is happening is that we've defined a dyadic chain, which treats a single argument as both the left and right arguments. In this case, the chain consists of "zip; interleave x and y", interleaving the input string with itself.
Someone already posted a 1-byte Jelly solution, so I hope it's not bad manners to post mine.
