Isn't two single beds in one room what is usually called "twin beds"/"twin bedroom"? That expression didn't come into use until early 20th century though, with American bias. Same with "king size" and "queen size" beds. These are American expressions of later date. The OP's question concerns an excerpt from Great Expectations, so mid-19th century British English. =) Admittedly I'm not a native speaker of English, but I'd say "double-bed" is rather unambiguously "one bed large enough for two people".