Split OA between CA and FA for RA.

After reading a lot of material on the benefits of OA (Outside Air) I am attempting to bring OA into the Return Air (RA) supply.

Currently I have 2 13' long horizontial OA ducts

ending in the FAU closet off the hallway (one high and one low).

I know that one should not route CA (Combustion Air) from the closet to the RA (just below the FAU).

But, is it possible to share the OA ducts and tap into the
ducts, before they reach the FAU closet and become CA? I can split the OA duct in two and route them to two different locations (CA and FA for the RA).

The other option is to re-purpose one of the two OA ducts as FA believing the other OA duct fulfills the CA requirement.

Note the door to the FAU closet off the hallway is not sealed so I am sure there is some air exchange between the closet and inside air.

I have reviewed the code and, due to our small size, we exceed all requirements for CA (see below).

50K BTU FAU is the smallest they sell? We only need 15K BTU according to online calculations.

701.4 Outdoor Air 2 permanent openings (currently have):
Where communicating with the outdoors through horizontal ducts, each opening shall have a free area of not less than 1 square inch/2,000 Btu/hour of total input rating of all appliances in the enclosure.

50K/2K = 25si

701.4 Outdoor Air 1 permanent opening (Maybe use this?):
1 square inch/3,000 Btu/hour of total input rating of all appliances in the enclosure, and not less than the sum of the areas of all vent connectors in the space.

50K/3K = 17si
4" exhaust vent = 13si

*** So why not use one duct for CA and one for FA?

701.9 Louvers, grilles and screens
... metal louvers and grilles will have a 75 percent free area.

Outside louver:
6x14 metal louver x2
84si each x 0.75 = 63si

7" horizontal duct x2 13' long = 38.5si each

Inside screen:
5x13 1/4" screen x2 = 65si each.