You can also have two PIC's at the same time.
Only one pilot may be pilot in command at any given time. You cannot have two PIC's acting as PIC in the aircraft at the same time. Two people may log PIC under 14 CFR 61.51(e), but only one may be the acting PIC. Logging time and acting as PIC is not the same thing.
Let's say the Captain is under the hood in VFR conditions, you are a safety pilot therefore you are a required crew member so you can log PIC and the captain can log PIC.
Being safety pilot or a required crewmember does not entitle one to log PIC. At best in this case, it entitles one to log SIC. In order to avail one's self of the opportunity to log PIC as safety pilot, one must be capable of and must be acting as the Pilot in Command. In order to do so, one must be capable and qualified, which in this case includes a type rating. In such a case, if one is rated in the airplane (meaning category, class, and type), one is logging PIC under the auspices of 61.51(e)(1)(iii): acting as PIC of an aircraft for which more than one crewmember is required by type certification or the regulations under which the flight is operated...91.109(b).
Your assertion, therefore, is incorrect.
As to the question of the value of the time, there is no such thing as "crewmember time." You might as well log passenger time, or time spent watching aircraft. How much experience do you have, son? Well, I spend a lot of time sitting on the airport fence watching aircraft land. I can spot a bad landing a mile away. And then I ride in them when I can beg a seat. I can tell when the pilot has screwed one up. It's one of my best things. Sometimes I ride up front and then I log it as crewmember time. I know there's no such classification or regulation authorizing or requiring it, but it looks cool, and...
Not what you want to say in an interview.
Is a SIC required because the aircraft is operating 135? No. It's the owner that requires the SIC. As the type certificate doesn't require a SIC, and the pilot is single pilot qualified, that leaves you in the cold with respect to logging the time. Even if the PIC were an instructor, trying to justify hours and hours of instruction in type would look ridiculous in your logbook.
The position really isn't upgradable for you, because no matter how long you sit there, you still can't log the time or show yourself advancing with recorded experience. Much better to find a job advancing with PIC experience in something you're qualified and prepared to operate, like a small piston twin. You need to walk before you run, get the necessary qualifications, and then move up.
Several posters asked why the owner doesn't type. I don't know the owner, and can only speak in generalities...a type rating is expensive. The SIC in this case isn't a required SIC but by the whim of the owner or operator. A type doesn't make someone safer, only legal to act as PIC. One can gain perfectly good training to act in the limited capacity as unnecessary SIC without the expense of adding a type. Owing to the lack of experience, if the position isn't upgradable, then the type would likely be a waste of money for the operator.
Conversely, if you can get the operator to type you, then by all means do so. Not only would the type enable you to start logging some of that experience, but it would also be of significant value in that it enhances your overall qualification.
Good luck!