I just did a simple experiment. I created one web application pool in my local IIS, then I made 2 web sites point to the same app pool. Web sites are NOT virtual. and of course, they function OK locally.
When should we start using shared app pools? And under what circumstances should you ever share an app pool between 2 web sites?
Do that for undemanding sites to save memory, separate app pool for small sites is in general memory waste.
Be aware that this way if your application pool recycles both application will be restarted, for example if you publish one site or just change web.config both web sites will be restarted (or more if you have more sites on app pool).
On shared hosting offten it's practice to put literally hundred of sites in one application pool :)