I am guessing the answer to your question most likely depends upon where you are at. To my knowledge, there is nothing illegal about tending to two patients at the same time. Hospital staff do it all the time.
That being said, I guess I am not really getting what you mean by this statement:
Are you wanting to write in one pt's PCR that you had another pt in your care also? The only reason I can see a need for that is if you are unable to do parts of your assessment because of it. Then all I would document is something along the lines of "Unable to obtain XYZ due to transporting second patient at same time." Of course, if one patient demands enough attention that you can not adequately assess a second one, then you shouldn't be transporting multiple patients.
Honestly, the only time I will agree to transport more than one patient in my ambulance is if I am taking two "walking wounded" or two patients that tell me something along the lines of "I don't really need an ambulance, I just need a ride there," essentially, just the patients that only need a ride. Now, that being said, if there are two patients that need more than "just a ride", but aren't critical, and there is a second tech available to ride in with me, if there are truly no other ambulances available, I will take two in at once. I will tech one patient and the second tech will be responsible for the second patient. However, since I work in an urban setting, the odds of not being able to get another ambulance are pretty small. It has happened before, but it was under extraordinary circumstances, such as a tornado or other mass cas incident.