What is some advantage and disadvantage to had 9-in W x 9-in L Stainless Steel Square Chimney Cap
Pros to capping are reduction of rain entering, prevents birds and squirls from nesting in chimney you can see when you need to have the chimney swept.
cons reduces draft , provides a place for cresode to accumulate ( further reducing draft and looks ugly when there is buildup) .
In some areas where builders cloth was required due to fire danger we installed wind driven directional caps to increase draft. I don't think the large holes in the photo will meet the requirement if needed for fire risk.
Now understanding there are 2 caps from OP comment, I think this will increase the area for draft and take longer for cresode to build up so this is probably a good idea. I can't see where it would hurt.