The main or only reason is, is to make sure your dosing equal amounts of each.
Triton recommends dosing all four the same throughout.
If you mix them, then the mix would have to be doubled, as I have come to understand it.
Another words,
5ml of part 1,
5 ml of part 2,
So to dose equal amounts of 3a and 3b mixed you would need to dose 10ml. (Technically)
Or keep it separate and dose 5ml of each.
YMMV
As ATL stated keep it all 1-1 ratio.
Mixing saves a dosing head...….
I keep them separate.