Gyes, we had such kind of issue: while installing 2 SSM-4GE modules to 2 ASA 5540 (Active/Standby) the firewall is splitted. That's my step:
1) Turn off standby ASA and plug SSM-4GE module
2) Power it On
After it was booted up failover relationships were broked and previously stabdby became Active appliance.
3) Turn off active ASA and plug SSM-4GE module
4) Power it On
After the it was booted up failover comes up and previously Active (on step 2) appliance became Standby. Everything is up and running now, but the issue was on step 2, I suppose becouse of distinct in hardware (when one ASA was on SSM reachest than another one). Still have no ideas why so happens and is there any way to plug SSM modules int ASA active/standby cluster without downtime.