You'll need to install each set to a separate directory under Everquest II/UI. For example...
Everquest II/UI/NightElf
Everquest II/UI/Laffs
With that done, you'll need to edit your eq2.ini file to point at whichever you want to load. The skinname should match the desired UI. For example...
cl_ui_skinname NightElf
You'll have to log out and change this setting each any time you want to swap UIs. There's a UI switcher utility that makes this a bit easier, but you'll have to dig it up yourself
Another alternative is to try the ingame command
/loadui NightElf
This option is very buggy and often crashes EQ2.