135 SIC what can I log

Its on our MEL, but with no SIC ops spec we are unable to use it. We have two Van's set up for single pilot only, blank panels on the right side. When we got our third we kept the autopilot but wanted to set it up for 2 pilots so we could hire guys without the 100hr make and model for single pilot IFR, and let them ride SIC. The FAA came back with the interpretation that we could go with a SIC program, but SIC's would be required in ALL 3 airplanes for ever IFR flight.