Originally Posted by mmff

This used to be easier said than done.

It's been a while since I last tried but in the past you could not, for example, credit the Zapping / Lisboa Viva card with 5 EUR* and then spend 1 EUR* on the subway and 2 EUR* to cross the river by boat to Almada. Basically, you'd need to spend all the credit in the first operator (in this example, the subway) and afterwards you could recharge the card and use it on a different operator of your choice.

I expect this to have been improved but maybe our local experts can confirm?

* Arbitrary values, I do not remember the prices for those trips.