The one going from the WP to the firewall flows into the heater box. The other one that goes to the cylinder head from the firewall flows into the cylinder head.

On my 240D the permanent fitting for garden hose flush is attached to the one flowing into the firewall. But then I move the hose and fitting to the other side flowing *into* the cyl head. Hose diameters are the same.

Douching and flushing the engine is something i do twice each year on first warm day in spring and last warm day in summer. Improved engine cooling and heater performance is quickly noticed.