Legality question...

If I have two 67 Darts and create one car, but only have one title... what are the legalities of doing this? Primarily I will be using the most (including frame) from the car without a title. Getting a title for the other one has been a big run around as it has not been registered or titled in many years and has changed hands a few times. I currently live in South Dakota...bought the car in Iowa... but am moving to Nebraska in a month and a half.