For pandemic usage when I need to filter my exhale, I disabled the check valves by popping the mask apart, removing all 3 valve flaps, and sealing up the output hole. This makes air flow through the filters bidirectionally. I've also seen a 3d-printed adapter for the 6100 series (I think) that attaches a third filter to the output. Separate input and output filters is probably a better way to go.