Don't look at the ratings the companys give you. Most are underrated. Instead look at things like stocking levels, planted unplanted ect...
Most people overstock their tanks so extra filtration is a good idea. Aim for up to 10 times water change or 900gph. 6 times per hour would be a minimum I would go with. But for messy fish liek oscars, goldfish etc... definatley aim high.
And spliting it between two is a better idea. That way you have better coverage over the whole tank and lessen the chances of deadspots with no current. Also if one breaks down things won't go south so fast.
It would be best to buy 2 of the same filter so you don't have to worry about all sorts of different filter media.
A big tank can be expensive to run.