Use ^\ to split like S. ie use P then \ while holding down control.
bind b break-pane
# Split another pane into this window.
bind B command-prompt -p "Select [<window>.]<pane> to join:" 'join-pane -s %1'
bind b break-pane
# Split another pane into this window.
bind B command-prompt -p "Select [<window>.]<pane> to join:" 'join-pane -s %1'
+# Split vertically.
+bind C-\ split-window -v
# Next pane.
bind i select-pane -t :.+
bind C-i select-pane -t :.+
# Next pane.
bind i select-pane -t :.+
bind C-i select-pane -t :.+