I have a licensed 18.2. I have downloaded 19.1.58. for my personal use. Can 18.2 and 19.1.58 coexist on the same laptop?

I do not see how to register my personal use copy of I do not have a serial Number or registration code.
I assume that the serial number is the one displayed in the Product Registration Program,

How do I get the registration Code?

Gail Thomas