I am often searching for the 50 off second room rates (i.e. MAJ/33F). A lot of times the rate on the first room ends up being higher than the lowest rate available for that same room category at the lowest rate/AAA rate/Marriott member rate. But usually the discount on the second room brings it back to equal or less. In the past three years I would guess 70% of the time the family rate is higher than lowest -- and the other 30% of the time it ends up really being 50% off what you would pay otherwise and being a deal.

For the hotel you searched, was the website showing you the total price for 2 rooms with the discount -- or was it displaying 2 rooms x the higher rate and in the fine print says they will adjust the price of the second room at check-in (as sometimes this is the case).

This is not just limited to Marriott. Accor is one of the best in terms of offering the 50 off second room, but for an upcoming Australia trip about 1/2 the time I searched -- 6 different stays/cities and probably a dozen properties I looked at -- I could just book 2 rooms at the lowest discount I had access to for the same price as 1 room + 50 off second.