A "double sofa" refers to a type of couch or seating furniture that is larger than a regular sofa, designed to comfortably accommodate two people. Full definition
Each suite has a double bed with double sofa bed. (pfitravel.com)
On the ground floor the property has a large living / dining room with double sofa bed, access to the terrace / private garden, shower room and kitchen with utility area. (rentals.holiday-home-rentals.co.uk)
There is also a comfortable double sofa bed in the lounge area. (wikiaustralia.com)