May just be two seeds that made two plants in the same pot, and they didn't notice when they were small.

You can pick which one looks like it is growing more normally, and cut one at the soil line. Or just leave them that way, the pot will limit the roots either way, and they already are growing as twins. You don't want a real tall plant anyway in a pot. Your choice.

Before I started my own seeds and I was purchasing tomatoes I would always look for the “doubles.” I would saw them in half down the middle and plant them separately. 2-for-1 deal ;).

definitely eliminate one of them. they will be competing for both space and nutrients and if you leave them as-is, you'll end up with two smaller plants and less production. more in this case is not better unless you separate them, and at this stage if you do, the plants may go into shock and take time to recover.

you can also just cut one off and root it in some water and in a few days you'll have another plant to plant w/out having to subject both plants to stress/shock from being split.