AFAIK - It invalidated the Budd Warranty if that was done, the cars were all designed with individual motors for that reason - although it may well have happened when eyes weren't warching
http://www.trainsarefun.com/lirr/lirr_rdc.htm about 3 or4 text items down the RH column
They were only lightly powered - they could be MU'd but that was because each car had its own motor - the warranty was voided if anything else was attached. LIRR obviously had done it, so the answer toyour question is "Yes", but at the owning companys risk
See also the post from Chris Mears at the bottom of this page http://princestreet.wordpress.com/category/lirr-2/
I didn't know about the warranty.
I did see a photo of a CN doodle bug pulling a baggage car, but don't recall where.
I'm guessing the couplers on the Budd car were for another budd car or for towing when broken down.
http://www.railroad.net/forums/viewtopic.php?f=63&t=67875
http://www.trainsarefun.com/lirr/lirrpingpong.htm
http://www.chessierail.org/lirr7045.html
http://www.lokpavillon.de/shop2/bildergross/279-5010.jpg
The doodlebugs were made by EMC (predecessor to EMD).
The RDCs were made by Budd, and could pull one car behind it, but it wore out the motors faster so Budd didn't want to fix them under warranty.
The RDCs were versatile for the fact that they could be rebuilt into unpowered coaches when the drivetrain became worn out and were too expensive to replace.
All the replies about the warranty are correct. The New Haven even tried easing the load on the RDC's by making up a 3 car train with an RDC1 at each end and a lightweight coach in between that was modified to run the MU connections through it to the RDC on each end. That didn't satisfy GM or Budd either as far as warranty.
Consider the RDC-1 only had about 550 horsepower. That's not a whole lot power extra to pull a second car.