As the guys are pointing out, the pins you see on the front of the processor are a socket connection for the lead and plug that come pre-soldered to the battery. The two rounded clips are retainers for the battery. It's possible that someone removed the old battery when it showed a fault so they could order a new one and they never followed up on it and that's why it's missing now?
Here's a sample image of how the 1756-BA1 comes...
It's not essential to have a battery, but very good practice to have a healthy one in case of brownouts or blackouts. You don't want to have to be reloading the program for every power loss or machine power cycle. Another reason why the SD Card is useful.
Why would there be two Logix processors in the same chassis?
Motion, yes. As you can only have one Motion group per Logix 5000 processor, there can often be the need to segregate into two or more groups.
You could also have an Enhanced Redundancy system where a failure on the first processor will failover to the second processor.
Or there could be two or more separate applications, within the same system, that require the same local I/O data (Owner, Input Only, Listen Only, etc.) all on the same backplane.
Or a second processor may just be used in the same chassis as a data collector or concentrator, and pass the data out to a SCADA system, say through a 1756-ENxT module.
For standard applications, there is no limit on the number of processors which may reside in the same Logix 5000 chassis, except for the limit on the power supply's 5v backplane draw.
Regards,
George