You shouldn't need to "combine" anything, assuming that your DXB-KHI-DXB carrier has an interline baggage agreement with United. I'm going to assume you're flying on Emirates for that leg?

Just give the first check-in agent (at IAD on your outbound and at KHI on your return) your itinerary details of your second ticket and you should be able to through-check your bag all the way through even though there are two reservations and two separate tickets. You would still need to check-in to get your boarding pass for the separate flights though, I think.

Welcome to FT!