When you said you use their calcium supplement, are you referring to the bag of dolomite that you mix in with the potting mix, or are you referring to the "snack" that's recommended to prevent BER? The snack is 1 teaspoon of calcium nitrate down the watering pipe every 7 to 10 days. That is supposed to help prevent BER.
I read one of your previous posts about dumping the boxes. Not sure if you do that every year, but I have read that BER seems to be more prevalent in the first year, possibly having something to do with pH. Potting mix is usually peat heavy, and peat is acidic, so the more the mix is in use from year to year, the dolomite helps raise the pH. Maybe it doesn't take effect immediately enough the first year to raise the pH in the fresh potting mix. You could test the pH and see if it's 6.3 to 6.8 or thereabouts. If you aren't yet using the snack mentioned above, try that.
I have only ever put one tomato plant in a box, along with a basil plant to deter hornworms. I would love to plant two in a box, but they get very large and probably fight each other for nutrients, as tomatoes have massive root systems. Also, crowding them hinders air flow on the leaves and could result in more diseases. Maybe some have had good luck with two, but it wouldn't work for me.
Another thought is that some varieties of tomatoes do better in certain regions than others. You might check with your local extension agent for suggestions on what works best in your area. But I'm really thinking the snack would do the trick if you're not already using it. I got lax last year and BER reared its ugly head, especially when the weather was really hot.
Just suggestions off the top of my head. Other than these, I don't have answers. I hope you find a solution. Gardening is rewarding, but it's a lot of work and time, and it can surely be frustrating when the end result is disappointing. Good luck!
