How can I switch between two generator inlets?
You're looking up parts with the wrong terms. What you want is a transfer switch. You don't need the kind that includes breakers, etc., since you're using an interlock; just a simple transfer switch.

In a transfer switch where the neutrals are bonded or overlapped during switching, the advantage is reduced arcing and transients during switching. The disadvantage of this situation is that the transfer switch can not protect your genset and other electrical system components from wiring problems, such as a neutral/hot reversal sending current along the neutral.

In your case, since you're using an interlock you will already be cutting off the electricity completely anyway before switching and starting your generator; neutral continuity is not needed. So you should use a transfer switch that switches the neutral along with the hots.

See http://ecmweb.com/content/switching-neutral-whats-controversy for more details.