Quote Originally Posted by MudRunnerTD View Post
Yes but the point of that is to control the shrinkage crack when the new pour dries and shinks. If a new floating floor is going down over the top with underlay then it not would really matter at all to be honest. Assuming it's the other way though, tile and mud bed scabbled out then add an pour to the existing slab level as a basic infill. Then new floating floor over that joint.

If still just use yellow tongue or ply.
Just putting up options for consideration @mudski .

Would only consider concrete poor for lower slab only if it was way out of level to obtain a good datum to come up from. Yeah preference remains yellow tongue or ply with use of battens if req'd.