Following on from this tread I've been doing a bit more investigation

I think the following is correct, but can't guarantee.

A normal LNB out puts one of the 4 signals - Horizontal High, Horizontal Low, Vertical High, Vertical Low. Hence why in a Sky+ box you need two feeds for dual record as the channels may be on different signals. Each feed requests a signal from the LNB.

There is a solution called dscr that bundles the 4 signals into 1 and sends them down one cable. For ease I'll call this a "dscr signal"

The Sky Q LNB appears to have the dscr combiner in built, so what it outputs is a dscr signal. Sky Q has the ability to split this to its different receivers and they are all configured to look for a dscr signal.

If you connect a normal Sky+ box to the Sky Q LNB it won't work because it's asking / expecting one of the 4 signals. However if you connect a Sky Q LNB feed ie a dscr signal to a dscr splitter and change the LNB type setting on the box, the box then recognises that each of the two receivers is looking for a dscr signal rather than each one asking for one of the four.

I have all the bits to test this, just haven't got round to doing so yet