My highly biased advice is to follow all, or at least most, of the advice above.
I like to have at least 3 putting putters of the same mold and same weight (within a gram or so). I like to use them all so that they wear in equally and I become accustomed to them all. I want to be able to grab any one and have confidence with it.
I will putt all, in succession, from the same spot. Gather 'em up, do it again. And again. As said above, this is to practice/refresh mechanics.
I will lay them on the ground in different spots, at the same distance from the basket. Pre-shot routine, set-up, putt. Walk to the next one. Pre-shot routine, set-up, putt. Repeat. Change distances and do the same. As said above, this is to practice/refine one routine, one putt, from a given, high percentage, distance. This enforces a routine and builds confidence.
I'll scatter them at random distances around the basket and within the circle. This drill puts confidence and routine into practice by forcing me to adapt to different distances.
Then there are drills with different stances, release angles, upwind/downwind/sidewind, around obstacles ...
My biased answer to your questions: Have multiple putting putters in the same mold are same weight. Mix up your drills to practice different aspects of mechanics, routine, and confidence. Why limit yourself to two. Wouldn't *one* be more realistic under this theory? I have, I think, 10-11 of my favorite putting putter. Just got two new ones today. Superstitious, I know, but when warming up prior to a tournament, I "invite" them all to try outs. One of them usually rises to the top and makes my bag for the round or rounds. If it fails me, I "bench" it and go with another. They all work.