The following may depend on your distro of Linux, but on mine each DVB card has a separate directory under /dev/dvb ie., adapter0, adapter1, adapter2
In each of these directories, there should be listed something like 'frontend0'
Check you have a frontend device listed for both your cards. I reckon you'll probably find one for the first card but not the second since this would seem to be what MythTV is complaining about.

That may help you pinpoint the issue better for resolution but I don't have a magic answer. I use 3 Hauppauge Nova-T cards and when the modprobe executes it finds all three. However I do have the problem on mine that the cx88 driver doesn't load up the frontends straight away from the modprobe (but that applies to all my cards, not just 1 or 2 of them). I have a line in one of my startup files that reloads the cx-88 driver again and then all the frontends appear and Myth finds them OK