PC is proficiency check. It is the checkride you must take to be current under FAR 135.293 to act as SIC. IPC is Instrument Proficiency Check which is a ride which "may" be required to act as PIC of an aircraft on an IFR flat plan under part 91.57.

What I was getting at is... if your company's opspecs require two pilots, and you are given a checkride annually, you can log it as SIC. This is what I do at my current job. Just a hint... if you guys operate "eligible on demand" then teh opsecs should require two pilots.