The two museums share Maurice A. Ferré Park and the car park beneath it, and they are a two-minute walk apart. They also suit completely different moods, which is what makes the pairing work.
Frost Science first with children. It is loud, hands-on, and exhausting in the good way. Two and a half to four hours is normal. Follow it with PAMM only if everyone still has energy; ninety minutes there is plenty.
PAMM first without children. Go at opening, take the galleries at a proper pace, then sit on the veranda over lunch. If you still want more, the Frost's aquarium and planetarium are next door.
On a rainy day, this park is the single best place in Miami to be. Both buildings are large, indoor and air-conditioned, the walk between them takes two minutes, and summer storms rarely last longer than that.
Combination tickets between the two have been offered at various points; ask at either desk rather than assuming.
Downtown and the Metromover loop are immediately to the south, Bayside Marketplace is a ten-minute walk down the bayfront, and the Adrienne Arsht Center is directly across Biscayne Boulevard.