Answer by David Chapman
There are advantages and disadvantages to either approach.
If you use both transformers simultaneously then you know they are both functional. There is a risk that the total load grows beyond the capacity of each individual transformer so that if the load is transferred to just one transformer it may be overloaded.
If you use only transformer A and keep B as a cold standby you run the risk that B, or some of the infrastructure supporting B, may fail when put into service. On the other hand, you can easily check that the single unit is not overloaded.
You need to be aware that harmonics cause excess losses in transformers, so simply measuring the load and comparing it to the nameplate is not enough. You either need to know the harmonic profile of your load and correct for it, or carefully monitor the transformer temperature.
Whichever redundant arrangement you select, you need to put in place maintenance procedures to guard against these potential problems.