There are specially made bags designed to piggyback (the 2nd wheel-less bag slips on top of the wheeled roller and balances nicely. I have this arrangement, made by Samsonite (but now discontinued -- I guess not that many people bought into it). Cost was quite a bit higher than for a regular bag, but the "docking" feature is fantastic, and it was narrow enough to fit in trains. Which can be a key consideration. Otherwise the real alternative is a backpack and not a duffel bag. I often go for trips around the world which cross the equator, so need both summer and winter clothing, and manage to do it with backpack (laptop plus light clothing) and expandable 22 inch carry-on (which is sometimes checked but usually both are carry-on). Of course the more important trick for a month long trip is to have the right clothing -- layered, interchangable and easily washed and dried. Laundry becomes a big challenge on long trips, if you are in a different city every other day.