Not a good idea. The heater flow is required to modulate the evaporator temperature (i.e. prevent icing). Also. the duovalve is on the outlet to the heater, not the inlet. At least it is on all the diagrams I have seen. By blocking a hose, you disable the system's ability to do something it is trying to do, and it will overcompensate (if it can) somewhere else, or just not work quite right if it can't compensate.

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'96 C280 (gone)
'97 C36
'05 C230k