Double spider farm - will it work?
Spiders and Cave Spiders are pretty similar. If you want, you can actually combine the two to get more xp. To combine them, it shouldn't be too difficult, just make the waterstreams (or however else you are moving the mobs) to connect together. Just make sure that the cave spiders can't fit out of the killing chamber, otherwise they will poison you! Use a slab with a trapdoor on top to keep them in.
So, my final answer will be that yes, you should combine the two. The more mobs, the more xp you will get in the end.