Both Manaus and Belem are listed as Oneworld Cities (on the AA map), You could fly into Belem and get a boat from the mouth of the Amazon (Belem) to Manaus. You could do the opposite, but IIRC the upriver traffice is closer to the edge and will get you a better view. If you are planning to stay at a jungle lodge or hotel you can get a discount on a package deal that includes the airfare from Rio. The airport in Manaus is very small and actually has very few flights. (and frequently I can fly to the USA cheaper than fllying to Manaus)