calendar synchronisation

We have 2 single chalets that are rented separately but can also be linked through a connecting door for a group accommodation but I don't know how to synchronize these 2 chalets with the double chalet on the booking site to avoid getting double bookings.

I know the I calendar but this is only for synchronisation with other sites

But I want chalet 1 and 2 synchriniseren with the double within booking site

Please let me know how to handle