It should stick together. Especially if you CPOP'd it to force gel with the top half.
Grace of From Grace to You, has a tutorial on welding soap together using a bit of moisture between the parts, and then CPOP to heat the parts and as a result, they will stick together. I've done it with a minor adaptation, and the soap never fell apart during it's entire use for handwashing and in the shower. But in my soap all parts were CP soap, so I cannot say positively that it will hold.
If you find that it does start to fall apart after you cut it, then I would suggest you moisten the surfaces you want to stick to each other, wrap the soap in plastic wrap and then CPOP as described in this tutorial:
Grace doesn't use plastic wrap, so you don't necessarily need it, but I used it to keep the parts securely tight together and the plastic wrap does not melt in the oven at 200° so I prefer my adaptation. But if you don't want to use plastic wrap, that's fine, too.
DeeAnna has posted a few times about a similar method for an entire loaf of soap, even after cut, wherein you put it all back in the loaf mold and CPOP again, which heats up the soap up enough to force gel in a soap with incomplete gel. It will also work to weld the broken parts together if you add the moisture between the layers.
Her post was about correcting partial gel, but it's much the same process except you would add the moisture to force the weld.
