Is a Heater Supply Unit the same as a Duovalve?

[rrobben]

All,

Thanks for listening! I'm having heating problems with my 93 500 SEL. The DTCs are:

E0 74 Heater supply unit circulation pup (A31m1), open or short circuit *)
E0 75 Heater supply unit circulation pup (A31m1), open or short circuit **)
E0 83 Heater supply unit left duovalve (A31y1), open or short circuit **)
E0 87 Heater supply unit right duovalve (A31y2), open or short circuit **)

Can someone tell me if the heater supply unit and the duovalve are the same part? Do they come together, or can they be replaced separately? I'm a newbie, and I need some help please!