On a more extended note... When you load into a world in single-player mode, even if you are using a different account, you will login as same player. Meaning same inventory/location.
But lets say you want to be off exploring a woodland mansion 8000 blocks away or in the End, while your GF wants to be working on a house at her base.
If you want to each do your own thing in same world, you will need to set up a local server.
You might need to do a bit of reading on how to properly set it up. You can either download 'vanilla' server software from minecraft.net, or you can get Spigot or CraftBukkit version. Once set up, you just need to click a desktop shortcut to start it.
When either of you want to play MC, you first start the server. Then, you start MC launcher, login with your account, and at main menu, select 'multiplayer'. You need to provide your PC's local IP (use ipconfig) and server's port. Or you can use something like LANbroadaster add-on to make your server appear as a LAN world.