No...it is not possible. When you use the DG Broker, the first thing you do in the DGMGRL utility is to issue CREATE CONFIGURATION. You can see from the doc for this command that here you define the PRIMARY DATABASE.
The ADD DATABASE command for the broker, adds a new standby database. You cannot add another primary.
The broker configuration is explicitly for one primary and all of the standby databases is supports. If you have another primary, you create a separate DG Broker configuration.