My SCCM 2012 RC2 environment consists of one CAS and one Primary site.
To make this work in a common SQL hotel with seperate instances you need to make shure that they don't communicate with the database on the same ports.
During the SCCM 2012 site installation you need to set:
On the CAS-Server "SSB Port" = 4022
On the P01-Server "SSB Port" = 4023
To configure this, go to the SQL Server:
Start SQL Server Configuration Manager
Go to SQL Server Network Configuration
Protocols for CAS-Instance - TCP/IP - Properties - IP Addresses - Set TCP Port = 1433 on all
Protocols for P01-Instance - TCP/IP - Properties - IP Addresses - Set TCP Port = 1434 on all