Yes, multiple logical servers can share the same IP address. including having a nameserver and multiple logical hosts on the same IP address.
You can have two nameservers on the same box, though it violates the spirit of the rules requiring at least two nameservers.
Having two nameservers on the same IP address violates the rules in fact, and I don't think I've seen this, though it might be possible.
The purpose of multiple name servers is redundancy. If you were to advertise that you have four nameservers, but all the nameservers were on the same box it would be fundamentally dishonest. If they were on different physical machines but the same pipe, it wouldn't be much better.