分割Tmuxウィンドウは、vimのnosplitbelowとnosplitrightの動作をシミュレートします。

からman tmux：

     split-window [-bdfhvP] [-c start-directory] [-l size | -p percentage] [-t target-pane]
             [shell-command] [-F format]
                   (alias: splitw)
             Create a new pane by splitting target-pane: -h does a horizontal split and -v a
             vertical split; if neither is specified, -v is assumed.  The -l and -p options
             specify the size of the new pane in lines (for vertical split) or in cells (for
             horizontal split), or as a percentage, respectively.  The -b option causes the
             new pane to be created to the left of or above target-pane.  The -f option cre‐
             ates a new pane spanning the full window height (with -h) or full window width
             (with -v), instead of splitting the active pane.  All other options have the
             same meaning as for the new-window command.

したがって、次の行を入力し~/.tmux.confてそれを取得すると、目的のアクションが生成されます。この-bオプションに注意してください。

# pane splitting
bind -r v split-window -bh
bind -r b split-window -bv