Hi,
To my understanding, starting from 8.3 software the ASA Failover pair devices wont require identical Licenses. Considering your ASA model, you will be running atleast software version 8.6.
With regards to Security Context Licenses to my understanding the Context Licenses on both units are counter together.
So you should have 20 + 2 Security Context available with the 2 ASAs Failover pair (one having the separate license and other with default license with regards to Security Context)
Have a look at this document
http://www.cisco.com/en/US/docs/security/asa/asa84/license/license_management/license_86.html#wp1353724
- Jouni