Hi Max1gk,
I would like to answer your questions
a) is it possible to do it with the version we have?
--> It's possible with SEP 11.x & SEP 12.1 also
Now you have an option to choose
1) Configure GUP at Site B,
Best Practices with Symantec Endpoint Protection (SEP) Group Update Providers (GUP)
http://www.symantec.com/business/support/index?page=content&id=TECH93813
Video’s created on Group Update Provider on the Symantec Connect website.
https://www-secure.symantec.com/connect/videos/group-update-providers-part-1
https://www-secure.symantec.com/connect/videos/group-update-providers-part-2
or
2) Configure proper MSL for both the SEPM's
About failover and load balancing
http://www.symantec.com/business/support/index?page=content&id=HOWTO26809
Installing a management server for failover or load balancing
http://www.symantec.com/business/support/index?page=content&id=HOWTO26807
b) how do we verify if the license we have allows us to do so?
---> Maintaining your product licenses
http://www.symantec.com/docs/HOWTO55300
Can a single license be split between multiple separate Symantec Endpoint Protection Managers?
http://www.symantec.com/docs/TECH164392
How to engage Symantec Licensing for help with product licensing issues
http://www.symantec.com/docs/TECH113612
Licensing enforcement rules
http://www.symantec.com/docs/HOWTO55135
Check this article for more details
https://www-secure.symantec.com/connect/articles/sep-121-and-license-concept
I hope it will help you !!!