My PC does not have enough resources to run two accounts in Unity 3D.
When I open a guest account from an already started session, it uses Unity 3D by default. How do I change that?
It is an interesting question. I've had a look at this, and it seems to be a bug somewhere. In my opinion, you should've been able to add a file called .dmrc to /etc/guest-session/skel containing
Where sessionname can be something like "ubuntu", "ubuntu-2d", "gnome", etc. But this doesn't work no matter what I've tried. There are several bug reports about similar issues, so this will be looked into.
I did come up with a solution for you, however, but it's really hacky and just hides the problem without fixing it. But it does work. Open a terminal and paste the following commands: (Make sure you get the whole commands, otherwise you'll get into trouble)
This just swaps the two sessions, so any user who has chosen to use Unity and not Unity 2D will also be logged in with Unity 2D until he/she chooses differently again. This also means that any new users you create will default to using Unity 2D.