Some people plant seedlings when they are 4 in. high.

I plant my seedlings when they are at least 1 ft. tall.

In your case I would wait a little.

Why you can not plant... BOTH seedlings ?

I was planning on waiting until they got a little bigger because they'd look really tiny in the 2' pot on my balcony and I think they have space to grow in their current pots. The leaves on the plant on the right look like a lighter green, or maybe the leaves are just thinner?

I thought container plants could do with all the space they can get, so I was planning on only 1 tomato plant per pot. I already gave away 6 other seedlings, these are just the 2 that I was keeping because I thought they looked the best. I promised to give the one I didn't plant to someone else anyway. How many tomato plants do you think I could have in a 22" diameter x 18" deep pot?

You can transplant tomatoes just about any time. I think the best way to look at it is when they get pot bound. Yours aren't quite there yet, but they're getting close. Yes, if you're going to put them in bigger pots, you'd really like to put them in ones where they'd look really tiny. If thing go right, they won't look tiny for long. Put both in that 2 foot pot, and you'll be good to go.

You might want to "harden off" before sticking them outside permanently. A few hours outside every day for a week will do the trick. Plants sitting on indoor windowsills aren't prepared for the wind, sun, and cooler temps of the outdoors. Even Stupice, which are quite cold tolerant, will benefit from that.

How many tomato plants do you think I could have in a 22" diameter x 18" deep pot?
^^^^^^^^^^^^^^^^
With a quick calculation, that pot has 24 gallons capacity. So if you fill it with 22 gallons of potting mix, that should be enough for 2 plants ( 11 gallon per plant). But the challenge ti managing the foliage in a close spacing. .
I would rather use 2 separate pots , each wit 10 to 12 gallons.

As with the question, which to one to plant first, I think it does not matter much.
Make sure that

1. The plants are hardened off before planting

2. Use soil less potting mix

Sey

If you were going to transplant to the garden, I would say go ahead. Since you are just going to pot up, they haven't outgrown their current pot. I would let them grow some more and start hardening them off to their permanent location. I also agree they would do better in individual pots. Good luck!

In my experience the younger plants acclimate better to the outside conditions. If you leave the pots indoors the plants grow faster but they'll be more stressed later.

Updated with better pics. I'll start hardening in a couple days since the temp unexpectedly dropped to around 5C at night. I'll also be going on vacation at the end of may, so I guess I'll need to mulch my newly potted tomato?