Well, you have to compute box fill individually on a box-by-box basis.
Unfortunately the 2 wires of pass-through must count as 2 wire counts in each box. They're conceptually pigtails, but each box computes separately.
I hope you're using steel boxes so you can reduce your ground wire count by 4 in each box. (let mounting screws and the nipple carry grounds between boxes and to each device).
I strongly recommend to have the "unrelated circuit" wires enter and exit the far end of the box, I'd even suggest a divider in the box to make it more clear to "the next guy" that these neutrals are NOT to be interconnected.
To compute box fill, you calculate
- the number of hot and neutral wires
- do not count pigtails which start and end inside the same box (they are free)
- Ground wires are "4 for the price of 1"
- 2 wire counts for each "yoke" (switch or receptacle)
- 1 wire count for all cable clamps.
You multiply this wire count by the cubic inches based on wire size. #14 wires need 2.00 cubic inches. #12 wires need 2.25 cubic inches each.
Then you know how big a box you need. Purchase boxes with that many cubic inches or more.
If you have a mix of wire sizes, count wires as what they are, but count everything else as the largest wire involved.