In most cases that I have noted, you can book up to two rooms at the senior discount rate, so long as one of the guests is eligible. I have done this several times, when traveling with a coworker who meets the age requirement. You do not need to have a qualifying guest in each of the rooms. If you did, it would not make any sense to limit the booking to two rooms at the rate. I have never had a problem doing this. In fact, I have never been asked for identification to prove that we qualify for the rate. I guess that just means that we look old.

The other question you need to consider is whether you want your parents to be able to earn points for their room. If you want them to be able to earn points, it must be in their name, with their Bonvoy number attached, and it must be charged to their credit card. You should, however, be able to make these changes at the hotel when you check in, so it is perfectly fine for you to book both rooms together at the senior rate. Then at check-in, you should have them switch your parents room to their name and their account and have them put their credit card on it. Based on my experience, you will need to double check whether this was all done correctly.