The old turf will prevent the new turf rooting down. You'll have to remove the old turf first. Good idea to decide why the old turf needs replacing, so that you can avoid whatever it was when you have the new.

Agree, you need to take off the old turf and prepare the surface beneath by digging it over, taking out any weed roots, levelling, walking all over it closely on your heels to get out any soft spots, then relevel, then lay your new turf. Turf needs to root into the soil beneath, so the soil should be relatively loose and friable for this to happen.