Dorothea and Dick lean out of their train window in the BBC adaptation Swallows and Amazons Forever!
The Ds are Dick Callum and Dorothea Callum. As a pair they are usually listed in that (alphabetical) order, Dick and Dorothea, often abbreviated to Dick and Dot in speech. Additional terms for the pair include the Picts and the Scarabs(PM).
It is not clear which of the two is older, but it can be inferred from context that Dorothea is an older sister to Dick. Dorothea's instructions to Dick are delivered with some authority and usually complied with, although Dick's recommendations carry the weight of expert 'scientific' opinion and are often deferred to by Dot. When Dorothea thinks that despite being one of the four elders, Peggy can not be much older than her (WH4), the implication is that she is older than Dick. The Japanese translations of the books name Dorothea as the older.
The Ds are a very loyal partnership, and cooperate to solve problems.