Thats called a "cut and shut" clip the rear half onto another shell. Its legal, just go with the car that has the vin affixed to its dash and register that one. The back half is just a parts car.
A buddy of mine pieced 2 fwd caddy coupe devilles together. One was kissed in the back, one kissed in the front. He used the paperwork and title from the one hit in the back since its dash was used and front was left intact.