I contacted one of Ariba's Solutions Directors to get guidance on your question. Here's what he shared with me:
“In time, suppliers will have access to a single network instance for all transactional data (today Quadrem and Ariba Network data). The transition of sellers to the combined network will occur in phases. The scheduling of the transition for seller users is based upon transactional complexity with the most straightforward exchanges transitioning first and more complex processes transitioning later in the project.”
Hopefully this provides the information you were seeking.
Ariba Exchange Community Manager