This is an incredibly confusing topic.
According to this website, Spain could make you surrender your British passport but you could still go and get another one as Britain says you don’t have to renounce your citizenship. Only Spain does.
Anyway, it seems to be - if you get a Spanish passport, don’t use your British one ever again in Spain or when travelling to and from Spain. Only use it say if you were travelling from Britain to a third country for instance.
There should be a proper statement from the Uk and Spain on this post Brexit if thousands of Brits are trying to get Spanish citizenship.
Hopefully, though, some sort of deal will be reached where Brits can still live in Spain without being treated like Non-EU citizens.