Don't know much about ACR cache, so I poked around (Windows 10). My system had both a "cache" and a "cache2" folder. Only 350mb in cache and 209mb in cache2. The file dates in both were fairly recent. So I launched Bridge and purged. Only the cache folder was emptied. The cache2 folder was untouched.
So I nuked them both outside of Bridge/ACR using Windows Explorer. No problem. Relaunched Bridge, opened several raw files in ACR, all is OK. The cache folder was rebuilt. No cache2 folder showed up.
Next, I set the max cache to 1gb, which is the minimum allowed, and tried to fill it up. I wanted to see if the cache2 folder would be created when I reached the 1gb limit. Couldn't do it. Turns out there is a bug, on my system at least. The max cache keeps returning to 5gb. Setting it to something else won't stick.
So, I have no idea why and when the cache2 folder is created. But I'm confident you can nuke it with no problem.