...until that traffic crashes into you or vice versa. I always treat traffic pointouts with high priority, they even saved my life once. From the crew and pax perspective (someone inside the airplane in potential danger), they're very important.
I am absolutely not saying that traffic calls are unimportant! I was making the point that higher priorities exist. If I have to choose between calling traffic between two aircraft that are legally separated and some other type of control instruction, I am going to issue my instructions first. After I have ensured the safety and separation of all my aircraft, then I will start calling traffic.
I got paid "to provide for the safe, orderly and expeditious flow of aircraft..." in that order. (I know you know that, I'm not trying to be contentious. I'm just trying to lay out my thought processes.) I was given a set of rules to follow which gave me the tools to do my job. I was trained to use my judgement to apply those rules, and sometimes (often) I had to use my judgement to prioritze what needed to be done. Some things are simply more important than others.And now, may I have a drum roll please...
I found what I was looking for, but it is not technically visual separation.
5-5-7. PASSING OR DIVERGING
b. EN ROUTE. Vertical separation between aircraft may be discontinued when they are on opposite courses as defined in para 1-2-2, Course Definitions; and
1. You are in communications with both aircraft involved; and
2. You tell the pilot of one aircraft about the other aircraft, including position, direction, type; and
3. One pilot reports having seen the other aircraft and that the aircraft have passed each other; and
4. You have observed that the radar targets have passed each other; and
5. You have advised the pilots if either aircraft is classified as a heavy jet/B757 aircraft.
6. Although vertical separation may be discontinued, the requirements of para 5-5-4, Minima, subparas e and f must be applied when operating behind a heavy jet/B757.
"Traffic, twelve o'clock, Boeing Seven Twenty Seven, opposite direction. Do you have it in sight?"
(If the answer is in the affirmative):
"Report passing the traffic."
(When pilot reports passing the traffic and the radar targets confirm that the traffic has passed, issue appropriate control instructions.)
You'll notice that there is no mention about flight levels in this paragraph, but it could only be applicable in flight levels because regular visual separation can be used below Class A.