what do you mean multiplex ?
if you have 6 tuners for recording + 1 for live viewing, then if minis or mobile want to access live satellite content they will each be taking up one of those 6 tuners, so they will not be available for recording.
Conversely if you have 6 scheduled satellite records you will not be able to watch different live satellite material on minis/mobile and will need to cancel a recording.
Erm, that's exactly what I mean by multiplex.
At present each tuner is 'reserved' for a certain function. This has the benefit that there are never any 'clashes' but the downside is that you can't record more than four things at once for example, even if you're not using minis or tablets and so there are several tuners sitting idle. The update will no doubt multiplex the tuners so they can be used for different functions. You're right in that this potentially means you wouldn't be able to watch on minis or tablets if you're recording too many things at once or, conversely, if you're watching on two minis and two tablets, you might not be able to record six channels at once.
Basically it's more efficient that way but can lead to clashes which are annoying. Virgin's Tivos have done this for a while but, as they only have three tuners, they don't even reserve one for live TV, so if you're recording three things at once, you can't watch a fourth channel live, you're restricted to watching one of the channels you're recording or other recorded or on-demand content. That's just annoying which is why I suggested Sky will likely keep one tuner for live TV at all times.
It's all speculation but I wouldn't be surprised if they did something like reserving two tuners for one mini and one tablet stream so they're always available, but made the others potentially available for recording should they be needed, so something like this:
1 dedicated to live TV
4 dedicated to recording
1 dedicated to live preview
1 dedicated to minis
1 dedicated to mobile devices
2 'floating' tuners which can be used for either recording two more channels or streaming to a second mini and second tablet