If you haven't seen these previous posts on the subject of double griddles, here are some links you may find helpful on the griddle question:

http://ths.gardenweb.com/forums/load/appl/msg0900353921214.html

http://ths.gardenweb.com/forums/load/appl/msg0722230524666.html

http://ths.gardenweb.com/forums/load/appl/msg011513426712.html

There is also this thread on chowhound:
http://chowhound.chow.com/topics/625879

This post was edited by JWVideo on Mon, Jun 17, 13 at 13:36

I believe Bluestar makes what you described. I've had my 36" with four burners and griddle for only about four months now, but can say I absolutely love the griddle! I have been using it more than the burners for just about everything. If I had room for a 48" I would certainly get a 24" griddle, they are so versatile and the heat is so even and controllable.

Don't know if you can get a 24" griddle with a 48" Blue Star RNB range but you can find out from the link below.

Here is a link that might be useful: Build your own Blue Star