In Part 91 you could log only when you are actually flying the aircraft, assuming you have the category and class ratings. I'm not sure if their 135 or 91. In Part 135 the Ops Specs can specify additional crew members. Even if it is a single pilot aircraft if you take a Part 135 SIC checkride in that aircraft you can log it as SIC time. I know at my 135 operator we flew sometimes as one crew member and sometimes as two. The SIC is a crew member and the revenue flight time in a day changed from 8 hours to 10 hours with two crew vs one. Part 135 the FAA certifies the operator and their operation and allows you to deviate from certain regulations.