Actually, you don't need to thin. I always plant out doubles. Why? Because one might croak, and I don't want to have a gap. But, but, they will compete with each other!! Yes, but the productivity will be equivalent to one plant with two stems. Do you pull off all but one stem on your other plants?

Beyond that, if you feel you must thin, just cut one and be done with it. Some preference to keep the one closest to the center of the pot, but no big deal, really.

You're probably going to have much much bigger problems later on. I'd just chill.

I would keep the right one. The leaning one on the right seems to be growing more normal. It is normal for them to lean, they don't keep growing vertically. The left one is probably trying to grow higher over the other because of the competition.

Does it really matter? Probably not.

You could separate them and plant both. They're not so sensitive that a bit of root damage will bother them.

Plant them both. Wait a couple of days and see if one is doing better than the other. Then the decision is easy of which to cut.

I always plant my zucchini indoors in pots and pick the best one or the one in center of the pot. Both these look good. I would just cut off one unless you need two plants, in which case I would carefully separate them and replant in separate pots. . Watch out when transplanting in the garden. The hot sun and wind can be tough on tender plants, and by tough I mean kill them. I use boards to protect the plants until they can take the outside conditions. This protects them from sun, wind, and hail. Here are boards on my parsley plants.

Ah, I see you have your own Sophie's Choice.