Below I copied out of Wikipedia. Midwest gets some single coaches, all else married pairs, but business class and cafe cars would not be married to each other, which is counter-intuitive to me since the cafe would not be the traditional barrier between business and coach class.
"Of the 97 cars, 68 are built as married pairs (two cars semi-permanently coupled with open gangways) while the remaining 29 are single (unmarried) cars with traditional gangways. Half of the married pairs (17) have a café car and an economy coach, while the remaining 17 have a combination business class/economy class coach and an economy-only coach. The business class and café cars have two vestibules each, while the economy coaches have one.[28] This arrangement—unique among Venture operators—allows trains to be sized to meet the travel demands of individual routes and allows business class seating and cafe cars to be added or removed. Among the 29 single cars are three cab cars with economy seating for exclusive use on Hiawatha Service trains."
I think the Northeast should have gone this way rather than fixed trainsets. Capacity cannot added if adding to consists is ruled out since frequency additions on host railroads is usually impossible, or the states just won't pay for them.