I haven't seen phones that use the two sims at the same time..... seen loads of phones which switch between sims etc though and of course the sim adapters. I read an article a long time back about that these types of phones where in development back in 2000 but the company got some serious negative "vibes" of the service providers who were afraid to lose revenue.

Have seen phones around which both function as mobilephone and dectphone so you call via the landline when you are within reach of your basestation.