Well there is a lot to tackle about your post, a lot depending on things that you have not told us.
You said that you are running a Ryzen 5 3600, but you didn’t mention what motherboard. If your motherboard supports it, you should seriously look into upgrading to a Ryzen 5800X3D. WoW benefits tremendously from the 3D cache on the 5800X3D and chances are very high that it would be a drop-in upgrade for your existing motherboard. CPU upgrades have the most potential, out of any component, to increase your FPS. You can compensate for a slow Videocard by using lower video settings, but there aren’t many settings to reduce CPU usage.
The 1060 3GB is a pretty low-end card. VRam less than 4GB is going to cause problems. When you run out of VRam, your system has to begin using normal system RAM to augment your VRam, which is much slower and higher latency. This will cause your performance to tank. So you really need to be cognizant of which settings you use.
You didn’t mention what resolution you run the game at. Resolution has a huge impact on VRam usage. If you are running at anything higher than 1080P, 3GB is going to be a problem. You could experiment with using a “Render Scale” of less than 100% to essentially decrease your in-game resolution. Make sure that you are only using anti-aliasing methods such as CMAA, and do NOT use any form of MSAA. Keep view distance sliders down very low (the fewer textures in your viewable range, the less will be loaded into VRam). I would upgrade your Videocard next, after your CPU.
16GB RAM is not a massive amount of RAM, but it should not be holding you back at this point. I really doubt that this is limiting you or that you would see any relevant increase by upgrading to 32GB. You may be misinterpreting your RAM usage. Windows will attempt to use extra available RAM to cache files for faster loading times. This RAM that is used as cache will be freed up immediately when needed by an actual program or game. I wouldn’t even consider touching your RAM until after you’ve upgraded both your CPU and Videocard.
Regarding running two copies of WoW at the same time, I can see how that would be problematic. You’re probably running out VRam very quick in that scenario, as 3GB is already barely enough for one copy of WoW.
If your motherboard has an extra full-length PCI-E slot, you could try adding an extra, cheap videocard into your system. In the WoW settings, you can specifically set which videocard WoW will use. When you use that setting, that copy of WoW will render the game using the videocard that you choose, and this is independent of which videocard your monitor is plugged into. Having your 2nd copy of WoW using a different videocard instead of both trying to share the same videocard would be an easy workaround to not clog up that 3GB on the 1060.
Also, I seriously hope that you are using an SSD, not a traditional mechanical Hard Drive. Otherwise I would make that one of your first priorities.