I've been running into a similar problem on my X3000. Two sims installed, different telco's. Upon boot everything connects as it should. When sim1 is configured then it connects to sim1 at boot, when sim2 is configured it connects to sim2. But only at boot.
When manually switching (auto switch disabled) between sims, more often than not, the connection cannot be established. Cell details are populated but the sim remains unregistered. Only after rebooting it will actually connect succesfully.