Hey it WORKED!
~/.config/openbox/autostart
gnome-terminal --full-screen --hide-menubar -- bash /home/pi/autostart.sh
and
~/autostart.sh (chmod +x)
#!/usr/bin/env bash
kodi-standalone
sleep 2
emulationstation
So on boot, Kodi launches. On first Kodi exit, the script will launch ES. From then on, ES does what it does when launching Kodi and returning to itself on exit.
Thanks for your help @mitu!