twin tuner recording issue

hi,
i have an twin tuner xtrend using open vix & cccam 2.3, 1 lnb but each tuner has it's own cable. if i try to record from more than 2 channels simultaneously ,then most channels become inaccessible, i can only record from 1 channel and watch a different channel, i have checked signal strength from both tuners to eliminate that one might be faulty and they both have identical results.
they are both set up as follow : simple / single /same frequency /no.
i have tried to do a manual scan for each one sseparately but that didnt resolve the problem.
any ideas please on how to fix this issue !

thanx for clarifyng that, i thought double tuner means multi-recording and being able to watch a third channel at the same time.the thing that confused me is that while most channels disappeared i was able to record from more than 3 channels and watch a 4th from the few available channels . bbc1 + 2 + 3 were still working and recording . but bbc4 wasnt working.