Why missile defence still fires two interceptors

Could modern missile defences use only one interceptor per incoming ballistic threat?

Normally no. A second interceptor is launched because the first can miss and there may be no time to recover afterward.

Defensive forces will continue to fire two interceptors at a serious incoming threat. Test success and manufacturer confidence cannot remove the possibility that the first weapon will miss in combat conditions. Waiting to observe that failure before launching again may leave no useful engagement window. The cost of expending a second interceptor must be weighed against the consequences of the hostile missile getting through. This is reliability doctrine rather than evidence that the interceptor is assumed to be ineffective.