I wanted to get the 45k points that were offered with the new card sign up that my fiance got (this is going towards the honeymoon). In terms of stipulations/restrictions, I never saw any wording or fine print prohibiting linking two cards to one account. There is obviously a field to input your PC rewards account and nothing that says you can't enter an account already linked with another card. Unless I didn't research it enough, there's no clear cut statement on this.
In my discussions with Visa and PC, it seems they may have formulated this restriction in my case.