Marriott credit cards simply give you 15 elite night credits. Until recently those did not stack but in the past couple of weeks that has changed very slightly. You can now get 15 for both a personal and business credit card, so a maximum of 30. That would get you Platinum if you stayed 20 additional nights. However, if you decide to do that be very careful the order you apply. There is a particular order you have to do it in order to get the sign up bonuses.
There are several ways of getting gold directly from a card (Amex Platinum or Marriott Brilliant cards). The only way I know of to get Platinum from a card is the Amex Centurion card. It would be cheaper just to check into a cheap Marriott for 50 days and check out though.
There are reasons that a body stays in motion
At the moment only demons come to mind