I don't get it. Why would you split? You choose the most profitable one and stick to it, until you find a more profitable coin, then you switch your entire mining power to it. Now, I remember during the time I was mining that the profitability of certain coins changed all the time, so I had to do a lot of switching but I never split my mining power. Maybe there are legitimate reasons to split but I never did so.
I have seen you have been answered by op and i do completely agree with this number 1 reason;
1) I have several different rig configurations (RX rigs, Vega, GTX 60/70/80, etc), and they all perform a bit differently on different algos. So I try to keep them pointed where they perform the best/better, for the most part.
Its just a matter of preference because you will really utilize those GPU on where they would be sufficient thats why splitting is always an option if you do really care about profitability.It might be sounds messy but its your preference on your end.