If you want to set up data sync between two subscriptions and one on-premises server that has both DEV and QA instances on the same server, and you have already generated two agent keys for the subscriptions but can only use one client data sync agent on the on-premises server, you can try using a single sync group for both DEV and QA environments: by using a single sync group to synchronize data between the DEV and QA environments.
Or you can set up a proxy server between the two subscriptions and the on-premises server to route data sync traffic.
Another alternative, you can try using a single subscription for both DEV and QA environments: you need to manage carefully the resource allocation and the control access but it can simplify the data sync setup and reduce costs associated with multiple subscriptions.
Overall, the best approach will depend on your specific requirements and constraints, including the size and complexity of your data, your network infrastructure, and your security and compliance requirements.