I'm not exactly sure what trainset you're talking about. Is it the San Diegan set? The one that came with a PA and really nice observation car? Lionel called it the "Surfliner".
If it is this set, then the two cars you'll need are in the following set:
Lionel 2 car add on for Surfliner
I think I found one of these on eBay:
Santa Fe 2 car add on set
This might be the set you have (check the names):
eBay 4 car set
I'm very familiar with the 1991 Super Chief set since I have this one. There are two add on cars for this set if I remember right. A full dome and an "Indian" series duplex sleeping car. Interestingly the Indian Series duplex sleeper matches a similar one Lionel made the same year in NYC paint. So maybe you want the 1991 set?
I'm sure someone will know better then my limited knowledge of these sets, but I think the 4 car Lionel Surfliner cars have fluted Budd style roofs, while the 1991 Super Chief set cars have smooth Pullman/ACF style roofs. So be sure to check the roofs on these cars so they match...if you want them to match. The Santa Fe ran both Budd and Pullman lightweight cars together all the time on almost every train, so it's not a prototype issue to run them together but you might prefer your trainset to match.