So, 4 days later and there's been no effective movement of the SG. How long do you wait? Is there a remedy, or do you just move to the next step?
And because I still haven't figured out the science-y part of this yet, what exactly does it mean that the SG has stalled?
I went back and re-read the whole post, so it sounds like you have two partially filled carboys with wine, and some extra in another container, all at around 1.002 - 1.004, pretty much not fermenting an longer. If you are OK with a slightly sweet wine, you can move on to the next step, which is degassing / stabilizing and clearing, but I'd wait a few more days before you do it, just to make sure that the SG doesn't continue to drop. Then:
Mix the batches together as best you can, as one is strong, the other weak, I'm not sure about the third vessel, but mix 'em up!
Fill your two carboys, but not all of the way to the tip top, leave room for degassing and clearing agents. Keep the leftovers in a third vessel.
Add/mix your KMS and Sorbate (you'll need sorbate since you're not dry) and degas the carboys using whichever method you prefer.
If you have sufficient wine left in the third vessel, you can try to save it, it'll need the right amount of KMS and Sorbate. You'll probably use all of your clearing agents in the carboys, but it'll clear on its own, just more slowly. Cap it and shake it a few times to degas it. Take the cap off and airlock it.
Once the carboys are degassed, add/mix clearing agents in both carboys, using wine from the third vessel, top up the two carboys to within 2" of the bungs/airlocks, and set them aside to clear. Put your "extra" wine in an appropriate sized container with little headspace.
Now you wait.
When the carboys are clear and ready to be racked, do so, and use the clear wine from the third vessel to top up.
Were I in your shoes, that's the path I would follow, assuming that my understanding of where you are is correct.