Just how much of a market is there?
I was a very satisfied Orange 'Line Two' user for many years as it gave me exactly what you're looking for: a simple means of separating Business and Personal numbers (Diverts, Bills etc.). With both lines active even 'Call Waiting' worked between them if needed.) It was only since my employer changed from Orange to O2 that I've had to join the masses and carry two phones around.
Ideally of course one could actually use two SIMs as you describe, giving flexibility between networks; I can only assume that 'reputable' handset makers are too cowed by the operators to allow this as I'm sure I'm not the only one who'd be prepared to pay a premium for the convenience of only carrying one phone.
Nokia are at least dipping their toe into the market with C1-00 ( http://www.gsmarena.com/nokia_c1_00-review-517.php ) but not only is this not a SmartPhone, more important is the following: "Only one SIM card is active at a time".
I find it quite frustrating as I know the technology is available, I'm sure there's a market and I have the readies waiting...
(Apparently Line Two is actually part of the GSM standard but only Orange ever implemented it.)