AHU/ACCU UNIT

Aa) Because that's the way it is designed. 2*100% gives redundancy in case one of the units fails for whatever reason.

Ab) 2*100%, obviously.

The more fully any individual unit is loaded, the better the moisture removal.

It's set up this way so there is no down time when a unit needs repair....the standby unit should be tested at regular intervals....it's not about efficiency, it's about critical application...

The units typically run more efficiently when near rated levels. The idle unit can fail due to lack of use. I would suggest that you alternate use of the units on a regular basis as in 1 week A, then one week B and so on. It should actually extend the life of the backup unit. The only issue I would see is there is likely to be autostart controls for the backup unit and if you alternate unit operation you would also need to be able to switch the autostart controls between units as well.

It is sometimes of advantage to advance the "run hours" [RH] of the principal unit in advance of the backup one, so that the backup one has rather less "run hours" than the principal. That way, the unit with the longer RH can be taken down for periodic planned maintenance [PPM] as a priority over the other, with the backup accumulating more RH than usual while the other is under PPM. That way, if there is a delay with the unit under PPM for whatever reason, the other can be used in the knowledge that there are spare RH to use up before the second overtakes the one under PPM; PPM on the units can be staggered to optimise the resources available with which it is to be carried out.

Which has nothing to do with unit efficiency and everything to do with resource efficiency.