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?
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.