A few questions, upper leaves, lower leaves or the whole plant shows the mottling? Do you know the pH of your soil, or at least a general idea of pH in the area. What have you already done for fertility? What was the source of the transplants, did you grow them yourself or get them from somewhere else? If you grew them yourself what was the source of seed? What variety are they, same variety for both plants?
It did start with the lower leaves and the upper leaves have still been mostly green. In general my soil pH is far on the alkaline side but this is a new bed, a small area I have the compost pile on for several years, including several years of Christmas trees, as I know the needles can help adjust the pH to the acid side. But, I haven't tested it, it was put into action with some haste this spring and things were growing very well so I really didn't worry about it.
The transplants came from a farm I know not too far from here, they sell at the local farmer's market. they are two different varieties, and there are two other smaller plants that I started from seed, direct sown, where the seed was saved from heirloom tomato's we used to get from our CSA farm. They are far behind the transplants but still doing ok.
My first thought is magnesium deficiency on your tomatoes. Causes can be low pH, sandy, rocky soils that leach nutrients easily. High potassium fertility can also contribute to magnesium deficiency. Cure is Epsom salts. Most places recommend using 1 tsp Epsom salt per quart of water and foliar feeding. Do a little googling to see some deficiency symptoms and see if that matches what you are seeing.
That was my first impression too. I did do a foliar feeding with epsom salt a day or two before I posted this but wasn't sure what I should expect to see in response or how often I should do it. At your suggestion I did some digging last night and it does look like that's what it is, or at least one of the things it is and the info says I shouldn't expect to see the yellowed leaves return to green but it should stop advancing. Still not sure if I should include a bit of the epsom salt in the water or not? I know the sulphate side of it will help with the pH imbalance...
I've been using a fish/kelp emulsion when watering and once in a while some Black Strap Molasses. Nearby winter squash are doing great, broccoli is doing great, except for the suffering the heat wave.
I also put a pepper plant, another transplant from the market, not far from the tomato's and it's been growing great, nearly three feet tall, but has had several flowers over the past several weeks but they disappear and not a pepper in sight. Don't know if it's tied in or nothing to do with the tomato issue.