Two shot

the shot is at 0.38

A Two shot is used to show display both character on camera at one time to show either a conversation and the actions or the reactions of the one or both of the subjects

In this case I believe it is used  to show the reactions between the character we can also see this is where the meet on 2 of the main characters comes from that of course this is the meeting of sherlock and Watson . this is a important scene for in the show