The Performax 25x2 has a single 4" port and you are correct in that it is totally inadequate. A 25" dual should have two. Unfortunately a 1.5 hp canister DC is the wrong tool for the job. You need to vent outside or have a good pre separator which in turn means a larger impeller. If you do need to filter through a cartridge when sanding, talk to Wynn Environmental for a recommendation on an easily cleaned filter as you will be cleaning it a lot. Dave