If these powerheads contain
mechanical media like sponges and
biological media like various bio-cubes, then they surely can act as filters themselves. They don't necessarily need
chemical media like
activated carbon / charcoal /
zeolite but if you use these as well, that's perfectly OK. If you currently don't have anything in the additional media box, use biological media there instead of activated carbon / charcoal / zeolite. You never have to throw away the bio-media, which ensures the
nitrogen cycle goes on uninterrupted. Activated carbon / charcoal / zeolite, on the other hand, needs to be replaced every month and in your case it would only take up the space for the much more important bio-media.
Now, you said that your 2 powerheads pump 200 L/hr (53 gph) and 100 L/hr (26 gph). That's a total of approximately 300 L/hr (80 gph). Your Dolphin canister filter does 130 gph and the 2 powerheads do 80 gph. Your tank is 60 gallons. Normally, you'd need 600 gph of filtration in that tank. Since you're using a 130 gph canister filter, you need only about 5 x the volume of your that with that filter, which is about 300 gph (10 x 30 gallons). As for the powerheads, if you intend to use them as the only additional filtering devices (next to your current canister filter), they should pump 10 x the volume of your tank, not 5 x the volume of your tank. You basically need 300 gph from your canister filter and you're currently getting only 130 gph from it. That means you need additional 170 gph of filtration. Your 2 powerheads give you 80 gph, so that leaves you with 110 gph still needed. If you use more powerheads or regular
HOB filters, that number of filtration should be doubled (220 gph). If you use another canister filter, that number can stay as it is (at 110 gph).
Sorry if it's confusing to you, but canister filters and external HOB power filters (or powerheads in your case) should be calculated a little differently in terms of
WPG required.