Hi,
Device wont send accounting logs sequentially to Primary and then Secondary and then Primary so on...
A device take a single device into consideration(the first one in the list). And will get authentication/authorization done from that very server and will send accounting logs to that server.
It will only send accounting logs to secondary server, if device determines that Primary server is down, and then it will switch for Secondary server.
I am taking out the fact the you have aaa accounting..broadcast configured. Due to which accounting logs are sent to all AAA servers in configuration.
Regards,
Prem