Hi!
fist we have a system landscape with multiple DEV Clients for one PRD Client BUT we do not use QGM but we do use ChaRM.
we use a lot of clients and transport from DEV_100 to QAS_100 to PRD_100 or DEV_101 to QAS_101 to PRD_101. And we use a central DEV client (001) to transport customizing that is applied to all Clients. Every Transport request that is released in client 001 is imported to QAS 001, 100 and 101.
as far as i can say its working, but the developer/customizer must always be sure what he is doing and what things should be distributed over all clients and what shouldn't.
regards
Alex