How to force tuner switching

[trantor]

Hello all,
Didn't find help on the forum yet.. Still reading

I'm in Egypt with a VU+ duo and following hot bird for the French channel.

I wanted to be able to record and look another channel a the same time. In some configuration, during recording, I've got a black screen on some other channel.
Then I install a second dishes + lnb on the second tuner. Means tuner A and B on hot bird. Same set-up


But the switch to the second tuner (B) is not automatic and I' ve still these black screen.

How can I force to switch on B tuner when I look a channel ?

Thanks for your help

Bruno