2 filters for a 40 gallon tank?

Hi Mei Roe,
Welcome to the forum! 

There is no such thing as over-filtering.    The more filtration the better water quality it is for the fish.   What you can do is to install a sponge pre-filter on the filter intake to prevent it suck in any more fish. It will also slow down the water flow slightly while increase the biological filtration.   At the same time a pre-filter can cut down the maintenance need for the filters by preventing most of the debris from getting into the filter.