But the wearer would have to prove that he/she didn't use the inner shoe without the outer shoe outside. On the other hand, if you can think of some way of making against "etiquette" of wearing the inner shoe outside.... But even then there would be some people who would use the inner shoe outside some of the time. Perhaps you can make an ugly or some weird design so that almost no one would want to wear it outside. I believe this idea would require marketing research.

I think your psychology is off. It is a matter of cultural politeness to remove your shoes when entering a dwelling, not a matter of enforcing compliance with a custom. The inner shoe would not be used outside as a matter of acting within the strictures of society but there is no way to force people to be polite.

Nice idea. Would save me the trouble of worrying about the (un)groomed condition of my feet, and is certainly more hygenic than the locker room-type conditions in public spaces where shoes must be removed.

Umm...it's a fine idea to be sure, but isn't the basic concept already called "socks"?

I do a style of skiing called Telemark and have some ski boots that have an inner boot that fits inside the outer boot. Maybe this could be adapted to a style of shoe. They also make a very light weight climbing shoe for rock climbers that fits snuggly to the foot and has a thin but very rugged outer sole. You could purchase a pair of these climbing boots before you you purchased a regular pair of shoes. Then you could size your street shoes wearing the the light climbing boot. You would only be about one shoe size larger then normal. It is worth a try.

People walk outside barefoot. My guess is that they wouldn't walk on certain places barefoot; they may, however, do so with this inner shoe on if this idea becomes a new fad. Therefore, it would be better to stick with wearing socks indoors and shoes outdoors.