You can buy combo tickets for the Museum of Tomorrow and the Rio Art Museum (MAR) online at https://museudoamanha.org.br/en. However, you have to visit the Museum of Tomorrow first (where you will be provided with tickets for both museums). Your second ticket will be valid for visiting MAR within the next seven days.
As with anywhere else in Rio, exercise caution when walking around. Don't wear excessive jewelry or leave purses on strollers. Cross-body bags are recommended.
Don't do this on a Monday since both museums are closed.
Rio gets hot and crowded from mid-December to end of February. It's the school holidays, so hotels, beaches and transports are crowded.
The museums are free on Tuesdays. The Rio Art Museum is also free on the last Sunday of every month. Kids five and under are free, and anyone under 21 are half-price.