if they are titled exactly the same way AND disney's administrative people realize they are owned by the same person (it's advisable to send them a heads up), then resales can be combined under one member number, just like adding-on directly.
the contracts will still be separate if you want to sell them or will them separately, but contracts with the same home resort can be pooled together to work as one contract at the 11 month window - and contracts with different home resorts can be pooled to work as one contract at the 7 month window.