(autoload "INERTIA" "INERTIA1" "INERTIA2" '("inertia"))
(or INERTIA2 (load "INERTIA2.lsp"))
Now all you need to do is autoload INERTIA. If you didn't want to modify INERTIA you could add INERTIA_load.lsp just to load the other 3 files and autoload it with the command call "inertia".