They have never weighed stateside to Portugal for me, however I've been subjected to weighing from Portugal (LIS, OPO) to the States (EWR).

They have been using total weight of the two pieces, some agents use 12kg (10+2), other agents for example at OPO use 10kg total (8+2), there's no reason to be found why a certain weight applies. It's pay up or if you have checked baggage allowance and you want to go back to check in, you can, but likely will miss the flight.

There has never been a lack of bin space for either routes, going or coming to the US, they do it to make money and honestly they seem to take pleasure in "catching" people and charging them 100+ euros.