Co-loading with Stand is really simple: In general, you just inject your other menu(s) of choice first, and then use the Launchpad to inject Stand.
Menus that have been tested
Impulse VIP 1.2.6
Disturbed 18.104.22.168 beta
You won't have any issues co-loading external menus, like Kiddion's Modest.
Limitations when co-loading
There is no consensus among mod menu developers on how to deal with other mod menus, and some even seem to ignore/deny the existence of others. As a result of that, most mod menus won't even load if injected after another one. Stand will try its best to mod an already-modded game, but there still is a handful of features that might not work in Stand, and you should use the first menu for, instead:
Vehicle Indestructible (aka. "Vehicle God Mode")
Protecting against and using crash models
Unloading when co-loading
If you want to unload menus in a co-loading setup, you should unload them in reverse loading order, e.g. if you loaded Stand last, it should be unloaded first, otherwise you risk breaking other menus.