<<There are a number of times that an ATC may be permitted to reduce separation in the "vicinity of an aerodrome", one of the provisors is that both acft have each other in sight, another is if the second acft has the first in sight (and will thus be able to maintain his own separation). >
AND - very importantly (and the first proviso in MATS pt 1) - "a) adequate separation can be provided by the aerodrome controller when each aircraft is continuously visible to this controller".
In this respect, there could be no requirement for either pilot to be informed of the other aircraft and it is not required for the pilots to have the other aircraft in sight. Eg Overflying a/c passing over threshold of runway at 90 degrees to the runway at, say, 2000 ft... departing a/c climbing out on runway heading to any altitude. These two are not traffic to each other, even if they are IFR, if they can be clearly seen by the controller.