Hi. Had to look up the Fx5. What a beast of a filter, recommended for up to 400g. However I usually halve the manufacturers figures for a filter in use with debris inside it. Even so, one filter should be plenty for your tank. Are you running the tank at 100% stocking or higher?
I run two filters on my tank, which is over filtered, because I like to clean them alternatively which removes the small risk of a mini cycle when cleaning a single filter. So I can see why you might want to run two filters.
My gut feeling says that they are not designed to be run in series and you might experience some syphoning/pressure problems. I suppose, failing a post from someone who's done it, you could try it and see.
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