Amtrak could do a lot of things if it "really" wanted to add another Viewliner sleeper to the Cardinal. It would just require TWO Viewliners for the TWO Cardinal train sets. Currently Amtrak is running 39 (max) out of the 50 Viewliners, leaving 11 in reserve for repairs, overhaul, substitute etc. However, don't think that Chicago, New Orleans or even NYP has extra Viewliners around sitting in reserve. Where Amtrak keeps these 11 Viewliners is one of the biggest mysteries to me (Hialeah, FL - near Miami is their home base). Amtrak could easily reduce the 11 down to 9 and still be fine. After all, they are not operating the Federal any more for overnight sleeping car service! And let's not bring up the fact that Sunnyside Yard is now scheduled so that no train sets coming "in" have a same day turn to go back "out" (i.e. like Chicago does with the Capitol Limited and Lake Shore Limited). Amtrak used to be able to do this with the Silver Meteor when it departed southbound out of NYP at 7:00 p.m. So in summary, again, Amtrak could "re-arrange" things so a second Viewliner could be had for the Cardinal.
If the Cardinal had two Viewliner Sleeper - yes the Diner Lite would likely be overwhelmed. However, the Cardinal once had the pick of two out of four train sets running as the Silver Palm! Recall that the Silver Palm was truncated from Miami to Savannah to become the Palmetto. Then two out of the four train sets of the Silver Palm (operating with Hertiage Diner, Heritage Crew/Dorm Car, and Viewliner Sleeper) were transferred to the Cardinal and the Cardinal then lost its Superliner equipment due to shortages of Superliner equipment after a rash of train wrecks (Auto Train, Capitol Limited etc.). But it did have and keep for some time the Silver Palm's Heritage Diner and Heritage Crew/Dorm car. Then finally it took to a model of the former Three Rivers by offering meals in the Horizon or Amfleet cafe/lounge. But could there be TWO Heritage Diners out there sitting at Beech Grove awaiting some refurb for the use on the Cardinal - perhaps. There should be more than just two for that matter! Amtrak was able to get four Heritage Diners up and running to get the Lake Shore Limited back on course. The other option would be to operate two Diner Lite cars on the Cardinal. PLENTY of these to go around.
So what is that is holding Amtrak back from adding a second Viewliner sleeper? Again, if it wanted to add another Viewliner sleeper to the Cardinal's train set, I think Amtrak could, but it is not going to. When the Great Dome was advertised that it was going to be on the Cardinal - the Viewliner was booked up by the end of August. Obviously this train is getting the higher end revenue passengers booking in the Viewliner sleeper. I took the last roomette on one of the journeys and I made my booking in August. Then - to add insult to injury - Amtrak advertises the Cardinal as a train between "New York City and Chicago." With the Viewliner sleeper selling out months in advance, will people choose coach? And if the three Amfleet II coaches get sold out as a result of the promotion, will Amtrak add another Amfleet II coach (likely not).
It's been rumored that Amtrak has been wanting to get a better "sightseeing car" on the Cardinal's route. However, this would have to come via a Superliner Sightseer Lounge Car year round, with truncation of the Cardinal at Washington DC instead of NYC. This would be a resoration of Superliner equipment to the Cardinal. It's also been rumored that the Cardinal had better ridership #'s when it operated with Superliners, as opposed to single level equipment. But then people will say that it provides a direct connection to New York City with single level equipment. Well if it had better ridership numbers, more capacity, and better amenities while a Superliner train set - I completely dismiss the NYC direct route. Amtrak could and has easily provided for a "cross the platform" change. Many passengers now have to do this "cross the platform station change" at various cities due to Amtrak's re-working or cancelation of trains. Long story short - Amtrak has claimed it just doesn't have enough Superliner cars to make the Cardinal a Superliner train set.
Personally, I think putting the Great Dome on the Cardinal is a test, as Amtrak may see that this route could be for both "tourists/sightseers" and those just looking for basic transportation. If they can get a blend of both, they serve two markets, rather than perhaps 1 at present.