CAT C12 OIL FILTERS

That's probably the CAT arrangement that Freightliner selected for use in that truck model.

CAT breaks their engine models into different prefixes, then within those prefixes, there are different arrangements.

 

Second filter is ussually a bypass in case the main filters plugs with debris. Maybe an option on this engine? Some smarter CAT fellers will share more.

 

Some of the 3176,s that were here had two filters side by side like that. The primary is a 1R1808, LFP4005 or 57792. The secondary filter is a bypass 1W8845, 51749 or LF777.
You can look up the microns of each. All of the oil that runs through engine is filtered by the primary unless a bypass opens. The secondary is a very fine filter, normally only has two inlet ports into it, and returns oil back to the pan. Only a small amount of the oil that runs through the pump goes through it.
The 777 replaced the big LF750B Luberfiner bypass filter. When the new trucks came out with those oil change intervals went from 8,000 to 16,000 miles.
You would probably need a nipple in order to install a second filter on you truck. I have never done that.
Things like that happen for different reasons. I have a Cummins with the 777 blocked off and a Spinner II mounted on the frame?