Translation:Both Saskia and Willem are from the Netherlands.
"Als" doesn't mean "and", it can translate to "as" or "if" for example. In this construction, similar to the English construction "as well as", the " zowel ... als" as a whole translates to "both ... and" or "as well as". You wouldn't take every word literally in the English construction either just understand the construction as a whole.
In English, "... as well as" is not a conjunction, therefore Saskia and Willem do not combine to become plural. We would say "Saskia, as well as Willem, comes (not 'come') from the Netherlands". Is that not the same in Dutch, so that the verb is 'komt' instead of 'komen'? (The problem is, so many English-speaking people get this wrong in English that I imagine some won't even know what I'm talking about!)
No, the Dutch construction is 'zowel x als y'. It's just the way it is :)