@Cormac0
If the guests do not volounteer information in their introductory message, I feel you would be in your full right to ask about sleeping arrangements.
At least that is my personal experience with my 2 bedroom/two single and one double bed. I have had all sorts of combinations like group of friends, two friends travelling together of same or opposite gender, mother and son, mother and daughter, elderly couples preferring a bedroom each etc.
I even had two male friends requesting a bedroom each, but only used the double, so probably more than just friends 🙂
In Scandinavia it is common to use single duvets even on the double beds, one for each person, so this keeps any arrangement flexible and easy to switch. The only extra linen needed for using both bedrooms would be one single fitted sheet.