Yes, and No.

You can signed up your megabonus point promotion, then switch your earning from points to miles, then you will see the megamiles promotion show up in your promotion center. You can then sign up for the megamiles promotion and start to earn miles. So you can take part in both megabonus and megamiles promotion within the same period. You can even switch between back and forth between these two promotion, but your stays will only counted toward one or the other, not both.

However, you cannot earn both points or miles at the same stay. So if you choose to earn miles, you cannot take points as arrival gifts. From my experience, many hotels don't even know that. Many times, FDA ensured me I can absolutely take the points as arrival gift when my profile set to earn miles. But the points had never gone through and post onto my account. And on the hotel's computer system, it never showed any error, or prevented FDA from doing so. They had never realized the points didn't post. It all looks good on their side. The points went out from their pocket into the system.

There was only one time, the bonus points post. What happened? it was a reward stay, and I didn't earn any miles. So the points went through. After that, I speculated it's Marriott's software system that has this glitch that prevents both points or miles from being post on the same stay regardless what Marriott's policies are.

There is another workaround. Some more experience FDA knew about the limitation, and what they did was to post the bonus points separately, i.e. not as arrival gift, and not related to the stay. Then, on other stays, I asked other FDA to do the same thing, but they had no idea how to do it. Then I gave up, and simply took the amenities.