Hi Anne, by oyster dish, do you mean the Sky minidish? If you do then there are both twin and quad LNBs available for that style of dish, but they're not cheap. As far as I know the mere connection of the second input is not enough to activate it, Sky do it via a software update and require £10 per month for the service.
I will move this down to the Sky Technical forum, as it will be more appropriate there.
We are always pleased to receive any personal tips and ideas about setting up travelling dishes, so post away, if suitable we can include them in our How To forum.