You probably should have a look at Spain's double taxation treaty with Switzerland (which would be available in Spanish somewhere in the Spanish tax website). These treaties tend to be similar regardless of country but it is better to check the specific one. How the swiss Quellensteuer in particular works within these treaties confuses me.

For a specific tax year (note that e.g. the UK does not start the tax year in January - I don't know about Spain) one of the typical things that counts for how to proceed is how many days you spent in one country or the other.

Though, if someone in the forum is more knowledgeable about these topics I would also be interested in the response.