Agreed on counterflow.
Re: separating the water heaters, the Vertex is a sealed combustion condensing unit, high efficiency. You would undoubtedly lose more energy with a second cheap water heater than you would with a heat exchanger, just in standby losses. Consider relative surface area and you can figure out why, and that ignores the flue and combustion efficiency differential which further stacks the deck toward the heat exchanger option (or operating cost differential in the case of electric DHW tanks).
If heat loss from a heat exchanger bothers you, insulate it and the single wall round tube water to air heat exchanger carrying heat to and from it
. The only other thing that matters is if your supply water temperature requirement is approaching the desired setpoint of the water heater, in which case heat exchanger sizing can be problematic since it relies on a temperature differential to operate...which is often incorrectly interpreted as "efficiency loss", which it isn't in any situation where the tank temp is fixed and the heating temp is consistently lower anyway. But if you need 125 degree water in your heating system, and the tank heater wants to be set at 130, that's a big heat exchanger you need to make it happen, or you need to turn up the water heater, which IS an efficiency loss. Especially with condensing water heaters you want to consider the tank setpoint carefully instead of just jacking it up. Deciding to add emitter to keep water temps down usually makes sense in those cases. If the water temps are high enough, it's time to consider a boiler that at least can stay in condensing range a significant amount of the time with reset control.
there are maintenance considerations to heat exchangers. But then again, there are maintenance considerations to separate water heaters too. Anything installed should be installed with maintenance in mind.