US is different to AA in this area. If it's one booking, one PNR, one flight number, same day, then it's one allocation based on end to end mileage, using US's mileage table. This applies if the aircraft changes in the middle. So ATL-SEA should be 210 TPs in First. You can use the online Great Circle Mapper, and double-check US' mileage if it is a 100 miles difference or less, which this isn't.

AA on the other hand, if you get off the aircraft, you typically end up with 2 boarding passes and each is charged separately (i.e. risking 60 TP + 60 TP). I caveat this in that I've not had a AA tramp ship recently, I often only realise it is a through service when they announce the "continuing on" information on board.

Clearly for two short hops AA's system is better, for a long one US works better. And I guess you can break tickets up if necessary.