If I put my old mac hard disk system drive with bootcamp in a USB caddy, after replacing it with an SSD, will it appear as 2 drives when plugged in? I'm going to replace my MacBookPro system drive / hard disk with an SSD. I will then re-install MacOS and Windows via bootcamp.

The original drive will be put in a USB caddy to be used as an external drive.

My system drive has bootcamp on it, so it has been partitioned HFS+ / NTFS

If I plug this drive as an external drive via USB will 2 (two) drives appear on my Mac Desktop? One for the MacOS partition and one for the NTFS partition?

(When I've put a partitioned drive in a USB external caddy and then plugged it into a Windows machine, several drives appear, 1 per partition.)
You'll see drives like as it was internally, just with a different icon.  You will even be able to boot from the external MacOS (press and hold alt-key when the system chimes on power-up), but this (boot from USB) will not work for Windows.  

But the files will be accessible as before.