I'm a bit confused. Number 1000 was/is a compound locomotive . That is it had one high pressure cylinder followed by two low pressuire cylinders in an attempt to improve efficiency.
However the word for a coach with both first and third accomodation is a composite
There are a series of books about LMS coaches by David Jenkinson and Bob Essery, which carried on MIdland design for a time.
The LMS Carriage Association may also be helpful.
http://www.lmsca.org.uk/links/carriage-preservation.php