They are getting root bound I'm sure. They need to be transplanted into good soil. They look too large to be in the small cups. Use good dirt and use some worm castings as fertilizer. My experience tomatoes need at least 10 gallons of dirt to thrive in a container.

That video doesn't address the well-documented phenomenon of Mg and Ca antagonism. Also, Mg sulfate isn't a macronutrient. Gary Pilarchek has some nice videos, but there's a lot of folklore in his content, and much of it is incorrect. Too much Ca causes Mg deficiency in the plant. Too much Mg causes Ca deficiency in the plant. BER is due to inadequate presence of Ca in the developing fruit. Too much Mg can contribute to this problem. While your garden conditions may benefit from extra Mg or be balanced enough that some extra Mg doesn't cause a problem, it's irresponsible to advise everyone, without knowing their soil situation or whether or not their plants are actually experiencing deficiencies, to apply epsom salts to their tomatoes. They could end up exacerbating a problem with poor calcium uptake and create a deficiency.

" is it odd to get 2 seedlings?" - Yes, it is odd. I always grow my own Tomato starts and it never happened to me. Are you sure you didn't plant there a pair of seeds glued together? " should I transplant one seedling from each pot into separate pots now?" - I don't know what you "should" do. If I were you I would remove (cut) the weaker seedling. "when should I transfer these seedlings into larger pots" - When they are well rooted and the roots start to be prevented from further development by the size of the pot. And, first of all, be careful not to over-water.

I bet all my veggies will be gone by the time I get back. The nights have been really cold for this time of year. That means the eggplants, peppers, okra and tomatoes will be a wilted mess because I haven’t been there to protect them. Oh well………..