You can't stack em, and they typically can't be combined with other similar options such as the PenFed discount. Though, there is one user here who apparently was successful in stacking them, so I do suppose it's always worth a shot.
When I picked up my truck, my dealer showed me in their system, exactly what they were dealing with. I actually had 3, 2 RAM cash (1000 and 500 or 750) and the PenFed (1000). They were all already tied to my name, in their system, some how - as I hadn't shared any actual codes or information with them yet. On their screen, if he tried to check more than one, it would just grey the other one out. Between all three. We obviously chose one of the two 1000$ ones. There were also a handful of other options they could have tried and applied to me, but I wasn't eligible, like the military and farmers discounts.