But I am still only getting the one deck. I have gone into Serato and to My Serato, and activated this controller. That aside, I don't see in the release notes that this is a version upgrade that enables Window 10 compatibility, and it does seem likely that the Windows 10 had been out long enough that 1.9.5 supported it.
But I am thinking maybe I downloaded a Mac version. My old zip file says 'Serato 1.9.5 Win' and the new file says 'Serato 1.9.6' without the 'Win.' I don't think this is a problem. I have downloaded the new version but wanted to check first. I have the same one deck problem now which was fixed on the old setup. Apparently, that was version 1.9.5 and now 1.9.6 is available. I loaded Serato DJ a while back but didn't immediately connect it and play around.
Now, I have a new Windows 10 laptop, the same MC4000 controller and Serato DJ. I believe I came here or possibly found my answer at Serato support. I remember being baffled when I only had one deck even though everything was plugged in. Some time back I had Windows 8, a Denon MC4000 controller and Serato DJ.