Can two users play on a Minecraft server using the same account?
1. Configure server online-mode off
If you are using a proper server you'll need to set "online-mode" to "false" in the server properties. You can ignore this step if you're using in-client LAN.
- Edit
server.propertiesfound in the parent folder of your minecraft folder. - Find and update
online-mode=false
The first player should now be able to connect fine as normal with the proper account name, or start the LAN server.
2. Modify displayname of second player
To get the second person connected, first go to your .minecraft folder. Find launcher_profiles.json and open it up with a text editor.
Near the end of the file you should find "displayName": and then your current display name. Edit your name to anything different, keeping the "s either side of it. Make sure that it's a valid username (only use A-Z, 1-9, and _, not over 16 characters, etc.).
3. Preventing correction of the file
This stage doesn't always seem to be required. If you're having trouble with your name being auto-corrected you may need to either disconnect your Internet, or make the launcher_profiles.json file read only after having edited your name.
To make the file read only on Windows: right click it, click "Properties", and then tick "Read-only" in the "Attributes" section of the "General" tab.
After you've done that, or if it worked originally, open up your launcher. You should see your edited name in the bottom right:
4. Start minecraft
You can now start minecraft. You can also connect your Internet or make the file writable at this point. Hopefully, you should now be able to join the server with a different name and without kicking off the other player.
Your name may be automatically be corrected if you start the launcher with your Internet connected and the launcher_profiles.json file not on read-only. You will also be unable to join proper online servers with the fake name, however you can use this method to set it back to your original name, or relog to have the launcher correct it.
Note to others: This doesn't allow piracy in any way. You still need to have a paid account and all of the game files downloaded. It's pretty much the same as /nick on modded servers.