It seems I have misinterpreted your question, what you are asking is if you can use only one of the two outputs or if you have to use them simultaneously.
The answer is that you can use any one of the two, or both at the same time without any problem as long as you are withing current specs (up to the max allowed current).
A small voltage drop is normal, can be cause by several factors like the wire resistance or internal psu implementation.
(my original reply below doesn't seem to relate to what was asked)
I would suggest to follow the specifications given without the assumption that when using a single output you can source more current that the manufacturer has specified.
The only place I've seen a combined output consumption effect the maximum current ability is with pc supplies but for the worse.
Here is what I mean, this is the specification of a random pc psu
The 12v secs are:
The total is 12v * 70A = 840W but as you can see the manufactures allows only 648 combined.
Here is another example
The Max specs for:
- 5v/3v are 130W
- 12v1/12v2 are 360W
- -12v/+5v (vsb) are 22W
If you add these you get 512W But the watts given for all of the above combined is just 450W, even the allowed peak consumption is lower than that to just 500W.