Double Science is taught by 3 different teachers at DCs school 9and every other one I've known.
Basically there used to be: 3 papers for Biology, Chemistry and Physics; B1, B2, and B3, C1 C2 and C3, and P1, P2 and P3. If you sat B1, C1 and P1 you got a qualification in Core Science, for double Science you had to also sit B2, C2 and P2, AQA also offered you further additional if you then did B3, C3 and P3. However if you did B1, B2 and B3 you could instead get Biology GCSE, and C1, C2 and C3 would give you Chemistry, and P1, P2, and P3 would give you Physics. All students (double and triple) learn the information for B1 and B2, C1 and C2, and P1 and P2 - and in all schools I've known these have been taught by separate teachers. The only difference for triple science is learning and sitting the B3, C3 and P3 papers, which is how some schools offer triple Science as an option.
Lots of science teachers only sat double Science and believe it is adequate for A'level study, however there is more of a "gap" between GCSE and A'level if only double has been studied.