On foot of a discussion on boards.ie, I did a quick test last night to confirm that the Octopus / DuoFlex set up supports multiple channel access on the same transponder/multiplex.
I managed to set up MediaPortal to successfully record 3 channels simultaneously and watch a fourth from a DVB-T feed (Saorview).
The channels I recorded were RTE Two HD, TV3 and 3e while I tuned TG4. As I've only got a single DVB-T card in my set up, giving me two tuners, this proves that the card is capable of accessing any additional channels on a tuned transponder/multiplex.
This is good news.
