Upon reading GNU Emacs Manual #rectangle register
When you are collecting text using append-to-register and prepend-to-register, you may want to separate individual collected pieces using a separator. In that case, configure a register-separator and store the separator text in to that register. For example, to get double newlines as text separator during the collection process, you can use the following setting.
(setq register-separator ?+) (set-register register-separator "\n\n")
I assumed that only the first line is enough as
(setq register-separator "\n]n")
?+ mean? and the extra
(set-register register-separator "\n\n")