You don't really have to have two separate service providers for that. You can use 2-port VoIP gateway (like those InnoMedia 3328-2 telephone adapters Net2phone sells with their service), and establish two calling plans under the same account, one with 212 area code, and one with 312 (going by examples you posted here). That would greatly simplify your billing, BTW.
But to directly answer your question, yes you can have multiple VoIP services on the same broadband connection.
This is still unknown. It depends on the provider. You definitely can do it with some combinations of providers and hardware.
That doesn't mean much to me. You'll register each device (or each line of each device) to an account with some provider. How they route calls to you is up to them.
Bandwidth (upstream, probably) is the only technical limitation. I have 384Kbps uploads at my house. Using a medium-speed codec, let's estimate 40Kbps (with IP overhead), I *could* support 9 simultaneous calls.
It's up to your provider.
Pick up to two additional phone numbers, with each number having a distinctive ringing pattern.
You can have as many Virtual Numbers as you want on a line or account!
Instead of just reading more about this, I recommend getting an SPA-2000
a VoicePulse Connect! account.
's what I finally did. It made the situation much clearer to me. Absolute worst case, you will have spent ~$100 on a great education.
(I don't think VoicePulse is the best provider for long- term use but their Connect! service is a good cheap way to get started.)
I think that the issue is, that depending on the cities involved, he may have to deal with two or more seperate VOIP providers to get service. If he is running multiple SPA-2000s (or other VOIP adapters) behind a NAT router, does he run into problems forwarding the necessary SIP ports in the router to more than one device?