I have the exact solution for you:
* For 2 houses, make 3 listings.
* Each house – listed on its own.
* Then the ‘3rd House’ [mother house, super-house, call it anything] as a separate listing for 20 ppl/ big groups.
Now link them together – if people book 1 House,
they cant book ‘the Big House’
If people book the BIG HOUSE,
they cant book 2 separate houses.
Price of Big House - make it bigger than sum of 2 separate houses,
Big House listing must be available 6 months in advance.
2 Smaller houses – 3 months in advance.
... and Bart,
pls be aware that people like you [with so many many houses] perceived by some people as rentier capitalism - are often blamed for house prices going up, housing shortages, environmental damage, breaking neighbourhoods [noise, problemss] etc, so in this forum we always advice each-other to give something back; donate to local schools, firemen's funds, build a duck-pond..anything to show that you contribute & we all care. Thank you.