It happens all the time. What I mean is that there is always a point, somewhere, where two objects in a particular field of vision can be made to "line up" in this manner. If you place two cans of beer at opposite ends of a table, duck down, and sight the two cans, you can move your head so that one is now directly in back of the other. Now think about two distant objects — let's say boats on the water. As they move it's possible that, from your perspective, they appear to approach each other and look to be on a collision course. But that's only the way they look to a distant observer — in reality they could be 200 yards from each other. And viewed from an alternate observation point, miles away, they wouldn't even appear to be close.
Any two visible celestial objects can be joined with a straight line. When this straight line is extended and is congruent with the line of sight of an observer, you can have a conjunction or an eclipse. Since the celestial objects are in motion and the earth is also in motion and since each individual trajectory is independent, these events are rare, but they can be calculated if the relevant orbits and velocities are known. Fixed as we are on earth we only see these events occasionally but theoretically you could map conjunctions in space by choosing an observation point in a direct line with the distant objects which are also in line.
I observed Jupiter and Saturn approaching each other but the sky was cloudy when they were in conjunction.