Sorry, I’m trying to convince you to use vim built-in features.
To make the copy/paste easy, you can open files in another Tabpages:
:tabe /path/to/another/file
Use gt or gT to switch Tabpages.
Or split the window to edit another file:
:sp /path/to/another/file
Use Ctrl-ww to switch Windows.
To split the window vertically, please use :vsp file
Update:
This is my .tmux.conf file:
# vim
setw -g mode-keys vi
bind [ copy-mode
bind -t vi-copy v begin-selection
bind -t vi-copy y copy-selection
bind -t vi-copy V rectangle-toggle
bind ] paste-buffer
# buffer
bind Space choose-buffer
I only use them when I need to copy terminal output.