setxkbmap -layout us,rs -option grp:menu_toggle

  grp:toggle          Right Alt
  grp:lalt_toggle      Left Alt
  grp:caps_toggle      CapsLock
  grp:shift_caps_toggle Shift+CapsLock
  grp:shift_caps_switch CapsLock (to first layout), Shift+CapsLock (to last layout)
  grp:win_menu_switch  Left Win (to first layout), Right Win/Menu (to last layout)
  grp:lctrl_rctrl_switch Left Ctrl (to first layout), Right Ctrl (to last layout)
  grp:alt_caps_toggle  Alt+CapsLock
  grp:shifts_toggle    Both Shift keys together
  grp:alts_toggle      Both Alt keys together
  grp:ctrls_toggle    Both Ctrl keys together
  grp:ctrl_shift_toggle Ctrl+Shift
  grp:lctrl_lshift_toggle Left Ctrl+Left Shift
  grp:rctrl_rshift_toggle Right Ctrl+Right Shift
  grp:ctrl_alt_toggle  Alt+Ctrl
  grp:alt_shift_toggle Alt+Shift
  grp:alt_space_toggle Alt+Space
  grp:menu_toggle      Menu
  grp:lwin_toggle      Left Win
  grp:rwin_toggle      Right Win
  grp:lshift_toggle    Left Shift
  grp:rshift_toggle    Right Shift
  grp:lctrl_toggle    Left Ctrl
  grp:rctrl_toggle    Right Ctrl
  grp:sclk_toggle      Scroll Lock