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. Other terms for the pair include the D's, the Picts and the Scarabs (PM).
Dorothea thinks that despite being one of the four elders, Peggy can not be much older than her (WH4), implying that she is older than Dick. The Japanese translations of the books name Dorothea as the older. Dorothea is the elder of the two (WH2).
Dick's recommendations however carry the weight of expert 'scientific' opinion and are often deferred to by Dot. When they are Signalling to Mars from the barn, Dorothea always felt that Dick knew best as he could think out things like this better than anybody although he could not make up stories about people. But Dorothea's instructions to Dick are delivered with some authority and usually complied with, eg (like Susan with Titty and Roger) telling Dick not to stand in the middle of the road (WH8). But when the cautious Dorothea was on the point of saying that Dick had better not go (on the sailing sledge) she stopped herself in time. She knew Dick's face very well, and one glance at it showed that at that moment it would not be any use to try to keep him back (WH16).
Dorothea lives in London as she had been seen off by her mother at Euston. Dick, coming straight from school, had joined the train at Crewe. For Dot: London last night, and now Beckfoot. (PM2)
The Ds are a very loyal partnership, and cooperate to solve problems.