I think the two festivals are working together about this question. So, I think they will be chasing the same names until they get something, but, if they can't, probably will go separate ways.
The affiliation between the two festivals is real and probably they signed a contract or something, like NOS Alive did with Bilbao BBK before. We have two examples that prove this: they sell tickets together to see the same band at both festivals and there is a contest where a non famous band can join both line-ups.
I guess Tame Impala could be a option... if they'll go separate ways.