Ich kan nur ser slecht deutch schreiben, so deshalb in English - zorry:
I am living in one the nordic countries where we have a provider, who is also using "Zweikanalton" on several channels, e.g. Norwegian in the left channel and Danish in the right.
For some time ago, I tried to create a small plugin, regrettably I could not achieve my goal without a patch in VDR. That is the reason why I never released it - Plugins are fine, but I am against patches 
The plugins isn't finished, but it is working in VDR 1.2.6 and 1.3.10. The 3 channels I'm using it for, is unfortunaly hard-coded.
If anyone can use this plugin for anything, you are free to do so.
monochannel.zip (packed on a Windows platform
In my humble opinion, this functionality, should be build into VDR.
Kind regards
DaddyOf007.