It will be much better if Real Madrid give up the La Liga title, because it will not be easy for them if they is still ambitious with La Liga while Madrid is still struggle for the UCL title.
Real Madrid's current situation is not the same as last season when they secured two titles in one season. In addition, their rivals have also developed from last season
Well if Real Madrid have the chance and opportunity to win the two leagues, why not because I already see Manchester City projecting the same pattern trying to get the premier league and at the same time pressing home the desire to lift the championships cup thos season.
Both Real Madrid and Manchester City have their own individual oppositions that may limit the success to clinch on both trophies but if they're managed to win one each it will be an achievement for both clubs since their the top clubn this season.