Show runner Steven Moffat runs both shows, they are both owned by the same network and they both have rabid fanbases. That said, the idea of a crossover episode between the BBC's "Sherlock" and "Doctor Who" has always seemed like an impossibility.
During a Q&A in Cardiff following the movie-length premiere of the new series of "Doctor Who," Moffat was asked about the chances of it happening. He admits that the possibility had been discussed and that if it was solely up to him, it would eventually happen as a one-off special. It's not up to him alone though:
"Look I'm going to come clean on this: I would... Go speak to Benedict Cumberbatch, Martin Freeman, Mark Gatiss and Sue Vertue, OK? They're all in the way. I'm not the killjoy, it's that lot. It's probably not going to happen. I know...What a bunch of killjoys I work with.
You know in some ways, I think Mark [Gatiss] has got a point when he says that however good you imagine [the crossover], it would be almost better in your imagination than it would be if the two grand old egotists actually met. They'd just both go off in opposite corners and sulk that there was someone cleverer than them."
The fan video "WhoLock," which cleverly blends Matt Smith's Doctor and Cumberbatch's Holmes into one short film story has pulled in over 3.1 million views since it premiered in December.
Source: Flickering Myth