All the website says is maximum 6 people, maximum 2 vehicles. There is no definition of "vehicle" nor is there any statement that only 1 RV is allowed per site.
As said before, every campground defines "two vehicles", "RV", etc., differently. In addition, each ranger and camphost makes their own determination as to what it means.
As I mentioned before, I have routinely paid the "extra vehicle fee" and had my friend in her chinook share my campsite. This is with full knowledge and approval of the people in charge (as I pay the extra vehicle fee in person to them upon arrival).
There is nothing wrong with sharing a campsite IF it's OK with the ranger/host - regardless of how someone online interprets the rules. The OP was simply asking what the rule is - and that is "it depends on the campground and the ranger/host". In the end, if you are told it's not acceptable, then get a second campsite or go elsewhere. If you are told OK, then enjoy.