My advice is that this is not the right way to go about creating a model of your own hand. There is no simple way to merge these two together, especially if they are not in EXACTLY the same position.
But let me answer your question first. Stitching would be the best, but you don't necessarily need the same number of vertices on each side, you just have to build the polys by hand (lol). There are some tools that make the process a little easier, like bridge (which is built into Blender, IIRC, tho you may have to turn it on in the add-ons part of the user preferences). To find bridge, press the space-bar and type "bridge." If it doesn't show up, it's probably not turned on yet.
A boolean would be the simplest option, but your two models have to be in EXACTLY the right condition for the boolean to do you any good.
If the two halves were in EXACTLY the same position (like two halves of a clam's shell) then a boolean union would be relatively simple, and would just require a lot of cleanup afterward.
However, if these were generated from two different pictures, and they're in two different positions, there's no magic way to just merge them together. Even if you were to stitch them together manually, you're going to have to do a LOT of fudging to make (for example) the index finger of one hand match the position of the index finger on the "other" hand. In which case, it's going to be really hard to guarantee that the final result actually matches your real hand. If you're going to do that much fudging, you might as well model it from scratch, the "traditional" way, and end up with a better topology.
If your goal is to model your own hand, this is probably the MOST difficult way to go about it ;-)
If your goal is to create a 3D model using 123Catch, then you need to follow their recommended method of taking pictures all the way around your hand, without moving your hand. This method will give you a single model, instead of two.