No, not always!

They could guess, they could choose whichever one you registered for first, there's lots of ways.

My point is that even if Marriott allows it, not all hotel programs (nor all airlines) allow stacking in all cases. There are some (non-Marriott) promotions out there which are not combinable with other promotions, per the official rules. (Sometimes that's just wording and doesn't really mean anything, but other times it means you really can't combine them, and if you register for both, it can be hard to know for sure which one you will get and which one you won't get!)

And even at Marriott, there is one kind of promo double dipping you can't do. While you can register for both a MegaMiles (miles bonus) and a MegaBonus / Unexpected Bonus (points bonus), you can't earn them both on the same stay. You have to choose between earning miles or earning points on a particular stay, and you only earn points bonuses if earning points and only earn miles bonuses if earning miles.

So, for example, if the question had been asked "can I earn the SpringHill Suites points bonus and a miles bonus on the same stay, the answer would have been no! It's only because the OP is asking about two points bonuses that the answer is yes.