Knowing related languages helps in learning
There will be plenty of similar vocabulary, grammar and possibly other similarities, too. However, the more you know, the more it helps, and this might not be a linear relationship; ideally, you would want enough of a handle on the languages to get a hold of the systematic differences between them and use that.
There are traps like false friends and different grammar; you will, for a while, be using a mixed version of the languages while using either of them, most likely. Though perhaps if you keep studying both or have a strong enough foundation in one, you can keep them separate.
Still, at least my experiences with the Scandinavian languages suggest that the help you get from a related languages is quite significantly positive.
In general, attempting to pass an exam with a minimum of study means lots of wasted effort from a learning perspective. In general, learning the subject matter will also allow you to pass any exam you are likely to meet, and gives you the benefit of having learned the thing as well.
The question of how easy a specific exam would be with your approach has to be answered by someone who knows that very exam (hopefully someone like that helps you out, too). In general, reading comprehension by multiple choice questions should be easier (though watch out for false friends), while writing words accurately and avoiding typos might suffer from influence of the language you learned first. But maybe not.