"There are six large bridges over this river."
Translation:E fölött a folyó fölött hat nagy híd van.
Just like how you would repeat a suffix (abban a folyóban), you have to repeat the postpositions too.
But yeah, that's long and we're lazy, so actually native speakers often shorten this structure the same way you did (I definitely do), which apparently isn't grammatically correct (yet...). So feel free to shorten it, but not on Duo and not in formal situations.
Speaking of formal, another option is to use ezen: ezen folyó fölött. But that's rare.