Hello,
The release notes for 8.4(2) should have what you're looking for:
http://www.cisco.com/en/US/docs/security/asa/asa84/release/notes/asarn84.html#wp535067
"For SSP-40 and SSP-60, you can use two SSPs of the same level in the same chassis. Mixed-level SSPs are not supported (for example, an SSP-40 with an SSP-60 is not supported). Each SSP acts as an independent device, with separate configurations and management. You can use the two SSPs as a failover pair if desired.
Note When using two SSPs in the chassis, VPN is not supported; note, however, that VPN has not been disabled."
-Mike