I have actually spent some hours watching rats come and go from their family hoses thru crystal clear ice and never seen one swim around the house just under the surface(within a few feet) not saying it never happens just that traps set there for me have proven far too unproductive to make. So I do not see the need to set one hi and low trap(but give it a try and see YOUR results). Only place I set baited BG near bottom is areas where rats are digging into the bottom for roots(most often found by plant remains just below ice. Feed beds are out of the game once ice forms(my point is that what keeps them close to house in open water and a prime set. Again this does not include "feeder" huts or pop-ups which I do set close(most have entrances that are not that deep and you are in their zone. More than a few ices of snow on the ice really reduces my catch I'm sure because of low visibility), clearly some away can increase penetration(I use lime foam pool float for bait as it is hi vis and doesn't discolor or freeze like carrot forgot in the truck). Lime green #1 choice, white second, orang third). I have tested all three together on stakes under ice and if any is gone it's LG. I have bubble trails go from one side of pond to the other all the time 1/4 mile +.
I now take a day or two(depending on now much I plan to trap) and if/when we get clear walkable ice(never go deep than your waist and know your pond bottoms) and just mark dens with fiberglass stakes and bubble trails with plastic marker flags. It often takes a couple of days fro trails to become well defined. Really active dens, runs often remain ice free and are easily found. Length of time with visible ice is not a given here. If marked, trapping can be done until ice gets too thick or snow covered(also overnight in MN some years, other two to three weeks).
Like bass ringed soft body lures mrats pelts bleed air as they travel under the ice and build up to breathable bubbles until frozen into deepening ice. Hence the more they travel the same shortest path the more reserve they build up. This is very easy to observe as mrats swims around the pond just under the that magnetic bee game- busy, busy bee. The habit of swimming near the bottom is a protection from aerial predators and most done near dens so predators can't easily ambush shallow den entrances. Travel up current in moving water can be easier near bottom and whatever is the driving force behind bottom edges. Once ice forms this habits changes to as you surmised easier travel just below ice, access to reserve air and higher visibility. Colonies I have set on the bottom in a couple feet thru vegetation need to be suspended to just below ice to remain productive(culverts seem to be an exception to this IME).
The test of any of this is for you to try it and see your results(all trapping can be different , different places. I'm still trying to quantify if setting my baited BG's(single spring 6x6's) jaws vertical or jaws horizontal would make a difference in my 15% sprung but empty traps. Which I would have to weight against my most convenient stake system for them.