Just a quick update.
Most of the Live TV problems were due to a bug with multiplex restriction code in Master, I have raised a ticket see https://code.mythtv.org/trac/ticket/12891
I have also tested with a single USB DVB-T/T2 tuner (Astrometa) LiveTV and scheduled recordings are working fine across Vbox and DVB tuners.
Note that the way Capture cards are provisioned has changed in Master (from 0.28/Fixes) in that the maximum number of recordings is now set in Input Connections rather than Capture Cards. I understand this is "work in progress" but there is a side effect which means the order of creation is important otherwise tuners may not be efficiently used. Unless things change it seems that a Capture Card needs to be fully configured before adding the next one i.e. Create Capture Card, Create Video Source, Create Input Connections, then do the next Capture Card. In 0.28/Fixes I was used creating all Capture Cards then Video Sources, followed by Input Connections.