"As for the SIC performing the duties of PIC under the supervision of the PIC is a very gray area. I would say that if you were not PIC typed in the aircraft, then the supervised PIC time, might not be valid. This regulation was written long before ICAO jammed the SIC type rating down our throats."
Read 61.159. It says "(4) 250 hours of flight time in an airplane as a pilot in command, or as second in command performing the duties of pilot in command while under the supervision of a pilot in command, or any combination thereof..."
There isn't a tradition in the US of having the PIC sign the SIC's logbook but if you have 600 hours in multipilot aircraft where you swap legs it shouldn't be too hard to come up with 250 hours.