Hi,
To make it clear. You have three options.
With SRM:
1. 2 vCenter servers and 2 SRM servers. All at version 5.1.
Without SRM (no test and no reprotect):
2. 2 vCenter servers 5.1.
3. 1 vCenter server 5.1 (so called single instance)
So, if you want test and automatic failback your choice is option #1.
Michael.