Actually this can be done in the Score editor, click-hold the link button and choose "Same Level Link" to make the button purple. Make sure you record onto a different track though. If you intended to merge into the same region, you'll have to merge manually, later. Otherwise the score editor works great for that!
unfortunately I needed it working at the same region , like almost all sequencers do ( cubase , pro tools , digital performer ) , , is possible to do it with garage band , , I can write to the garage band seeing both the old and new notes on the same track ( both -score view and piano roll ) , but I can only view one score per track , if there is an option to see all tracks in score view in GB like logic , my problem is solved . thanks