What's the most efficient way of locating someone based off knowing their location relative to someone else?

A simplified version of having 3 people and you know the distance of A and B and know C only in terms of closer or further away from A and B, would also be interesting to know if only having 2 people is too difficult.

I am not sure I understand the question. Are you only interested in seeing if B is closer or farther away? Dimming or blocking of the target position would do that.

If you want to know the distance and can move from a known spot to another known spot you can record the viewing angle and use the parallax equations to find the intersection of both lines and thus the location.

Sorry I’m trying to pin point where is B location

If you can move A anywhere along the circle and know the angles, you can find B in two moves. The base of the triangle would be 2 km and the angles to B would be used to determine the distance. If you measured at the extremes when A was furthest to the left and right the distance would be the tangent x 1 km

I don’t know Bs or As angle to me

GPS with a radio link.

Plot them to a map (electronic or paper) and see.