Have two Minecraft players coexist at separate times in the same local server

There's only one computer in the house. My brother and I play Minecraft on it using the same account. Up to this day, I play on my world, and he plays on his own (which just so happens to be the same seed, thereby the same world).

I want to know if it's possible to turn this world into a local server (or just recreate a world and make it into a server) and then have make it so that he can log into it and build things. Then he logs out, and I log into that same server, and see the progress he's made, and make my own. In essence, we wouldn't be playing at the same time, but we would coexist in the same server. Is that possible? If so, how?

The simplest way I can think of to do this is having 2 player.dat files which you switch out each time one of you plays. Of course, this would be a bit of a pain to do manually, but writing a script to switch quickly shouldn't be too hard.

This would be done in a singleplayer world with the same minecraft account.