Basically you should try mimicking all of the physiological factors of taking a shit before the person actually takes a shit, so that everything is ready to go.
2 girls simultaneous shitting on a guys face
Hydration is an important component of producing a bowel movement, as well as increasing blood flow.
How to Get a Girl to Shit on Your Chest
Touching his or her erogenous zones would probably help, too.
The more ways you can simulate everything having to do increased blood pressure. Work on approaching shit together from a leveled, balanced perspective.