Obviously you can't have two keys that do the same thing when both programs are listening. Tmux is going to get it first. You didn't specify which set of key bindings you don't want to re-learn, but you have to do something different because the same key is used for both apps.
If you only use it occationally in
less you can just pass the real thing on by hit Ctrl+BB (that's Ctrl-B twice) in
tmux to pass on a single Ctrl-B to the app in the current pane.
If you want to change the
tmux binding to be something else (say like Ctrl+A like
screen) you can add this to your `~/.tmux.conf file:
set -g prefix C-a
If you want to use something else for
less, the G key is usually bound to go to the top of the file already, so no changes necessary. If that change doesn't suit you, you can rebind keys using