You have a typo in the second command; you need to escape the semicolon. See the example in man tmux
:
bind-key R source-file ~/.tmux.conf \; \
display-message "source-file done"
As the manual goes on to explain:
Multiple commands may be specified together as part of a command sequence. Each command should be separated by spaces and a semicolon; commands are executed sequentially from left to right and lines ending with a backslash continue on to the next line, except when escaped by another backslash. A literal semicolon may be included by escaping it with a backslash (for example, when specifying a command sequence to bind-key).