Thanks for all the various inputs.

So if I understand yall correctly, I ought to get points from relevant cash incidentals arising from the points-only reservation as well as the normal accrual of points from the room rate of the cash-only reservation, yes?

And to ensure (as far as possible) the proper credit of nights/points, I should request the hotel to keep the respective folios separate (and of course where possible to keep the same room), check out of the first reservation and refresh the keys for the second booking.

But as Margarita cautioned, it seems prudent to bear in mind that some cockup could happen and that I might have to submit a missing credit claim, hence be sure to have the relevant folios.