Only Samsung Duos, there are two models, one sliding n one candybar shape are the mobile phones with Dual standby from a major manufacturer,
There are a lot of Chinese phones with this feature, you can check in Al Sadd mobile shops, most of em has it.
There is one Eten Glofiish Dx900 coming, that is a Pda with dual standby, and 3G don't know when it is goin to be available in Qatar.