Here's what I am doing this fall. Not my idea, I've seen it many times on the internet and some have made reference to Michael Palmers system that is similer, I believe.
A little different than a split. Divided 10 frame deep box's. You don't really make a split so to speak, you have the same size hive, same amount of bees and frames. Just divide with a queen on each side. Let them mingle above a queen excluder. If one queen dies overwinter it is said they will move to the other side to cluster with the other queen.
I am doing this to many of my hives due to a surplus of really nice queens.
2 divided deeps with pollen patty above brood nest.
Add third box for fall expansion. Queen excluder on top.
Some of the queen excluders need a small pan head screw in the center to keep it tight to the divider.
Bees can mingle above if the two queens are established in their hives and laying well. Here you see they share a top feeder:
Blue tape has notes about each side and each queen
Perfect is close enough when dividing the box's. You will invariably have some irregularities on wood at some point. Then use the old carpenters saying;
A little putty, little paint we can make it what it ain't ! In this case I use clear silicone to fill in any gaps.
Always staple your bottom first. This must be perfect to avoid bees sneaking through. You can calk the top easier than the bottom if necessary.
I use 3/8 plywood for my dividers. If you trim of the shoulder on two frames you can get five on each side if they are not drawn out too fat. If you use solid wood for the divider beware it can shrink after you install it and leave a gap at the top or bottom. Plywood is more stable. I'll be overwintering at least 35 hives just like this + my full sized hives. The love the vertical hive set up.
Here's my bottom board. Fully screened with a divider and entrance on each side. This is for mated queen use. For mating nucs I have an entrance on each end. Screened front landing deck for my wet winter climate.
Divided mating nuc ready for second box: