There might be an interference if the straps are rubbing on each other, like in some seats where the LATCH belt is just in the same belt path as the seatbelt. At least that's the best explanation I've ever heard. The belts stretch at different rates and it may make the seat perform in a less-safe manner. But there's actually some differing recommendations depending on the vehicle and the seat manuals. Some actually do allow it, at least one car even recommends it. What car and what seat are you working with?
In general, it's just double the effort for no extra safety, the seat will work fine with one system or the other when it's installed properly (ie, not installed in a seating position that doesn't allow LATCH, or not over the weight limits listed on the side of the seat, using the top tether when forward facing, etc).