Pick your relative position (say, you are on the left of your buddy), then stay side-by-side, arm's length. If I glance to my right and can't see my buddy, our positions are bad.
Practically speaking, when I dive with really new divers, just post cert, I like to be there in case they have a buoyancy issue. Also, if buddy has a random issue, like a cramp, if we're pretty much even they have time to signal me even if they stop dead in the water.
Doing side-by-side also should force the stronger swimmer to go slower, a good thing -- many of the things to be seen require slow speed, not a sprint.
If my buddy is pretty experienced, we'll separate a bit more, no big deal. With my regular buddy we'll tend to be within 5 or 6 feet max.
"Pretty experienced" isn't purely number of dives. Some people have poor buddy awareness even with a large number of dives.