With an established filter already running you have options. You could do either one, let the new filter alone to develop beneficial bacteria over time or seed it with some media from the established filter to speed the process. Personally, I'd do the latter if there was a real need to add filtration in the first place.
Your beneficial microbe colony won't outgrow the available nutrients, either way. In other words, if filtration, substrate, etc. weren't enough to support sufficient beneficial bacteria, an added filter will improve that, but if your bacteria colony population was already in equilibrium with available nutrients, more filtration won't increase it beyond that equilibrium. The new filter would eventually just share whatever beneficial bacteria is supported by the available nutrients, oxygen, etc. Doesn't mean more filtration can't increase water polishing capacity.
I'd say you could be increasing capacity for additional bio-load in the future, but if you're already heavily stocked, that's probably not an option.