Actually you can. The knocking sound emanates from the target (i.e., the door), and the range of knock is 60 feet. So you can have the cleric* put silence on the door, and then the wizard stands outside the radius of the silence effect and casts knock.You know that you can't cast knock once silence is in place, don't you?
At which point, you have expended a mere 2 second-level spell slots in order to... uh... open a door. Is that a deal, or what?
*Silence is not on the wizard's spell list. Bard is the only class that can manage both spells unassisted.