Chinatown mate.

I've seen loads of dodgy Chinese knockoff's that have dual sim as standard. The phones are utter shit.

$127.50 - N-95+ Triband 3.0" Wide Touch Screen GSM Cell Phone*EMS - Cell Phones

Was on an old Nokia but I used to have 2 numbers on the same phone with 1 sim on Orange. The service was called "line 2" and you had separate ringtones and mail boxes and separate billing too. Don't know if that would meet your requirements.

Interesting, I'l look into that.

PS