Do well? No, sorry. They will compete with each other for water and nutrients thru out the season. Plus they will be additionally stressed for awhile as they now try to develop the feeder roots they need since you skipped the initial transplanting. Plus you will likely have over-crowding and poor air circulation issues. Cut off the extras close to the soil level so each plant is on its own and as they try to resprout keep them cut back.

You have learned a valuable lesson - why plants are started in small containers and transplanted to individual containers as soon as they develop their first set of true leaves. By trying to avoid the initial transplanting the trigger they need for fibrous feeder root development was never activated and they now have to play catch-up.

Dave.

"For example, for tomatoes, having two plants planted together isn't
really any different than having big two stems come out of one plant."

I would agree Dan IF you were to prune each of those two plants to a single stem rather than multiple stems and IF you could ensure that each plant got equal nutrients and water to their individual root systems. Those are big IFs.

The difference is that one plant has one root system, one circulatory and nutrient distribution system. Two plants in the same hole do not. The roots of the two plants may be tangled, but they aren't fused. They are still supplying only 1 plant, still stealing from the other.

Plus you have doubled the other stress factors that even a single plant has to cope with.

So, if as you say, 2 or 3 plants in the same whole will still produce the equivalent of 1 plant in that hole why not simply cut it back to one plant? What is gained by leaving them all vs. what is potentially lost due to over-crowding and stress?

Think of it as a 3 legged race. Two folks with their middle legs tied together can't run nearly as fast or be nearly as successful at completing the course without falling as one guy can do it alone. :-)

It isn't a matter of "doing anything wrong" but of now fixing something that wasn't done really well to begin with.

Dave

IMO, if there is enough soil/space, 2 plants to getter can co exist and each will do its own thing. All they need is moisture and nutrients. But the bigger challenge is trying to manage the top/foliage in close proximity. A lot of plants are grown in clumps that they don't need to be supported, like cucurbits, potatoes (Tomatoes cousin) . I have always held the view that tomato spacing is mostly to accommodate the support and head room.

To answer the question

" Is it too late to pinch off the extra plants?"

My answer is NO, it is not too late. By pinching off one( from way bottom) its roots will die . You do not need to separate UNLESS you want extra plants. You can even separate them, if you want. Put the whole thing in water and keep shaking and gently pulling them apart ( while shaking). Plant them right away. Use some fine potting soil in the planting hole. , flood the transplant

Tomato plant is very forgiving when it comes to transplanting.

Sey

Thank you all for your comments. I will probably cut them down to one plant per hole as that is what I had originally thought I would have to do. I didn't realize when I started them, that transplanting from a small container to a larger one was really necessary as I had never started plants indoors before. (I still don't understand why, since they don't get transplanted if they're started from seed in the garden.) Looks like I still have a lot to learn. I appreciate your input.

The idea is that by transplanting into a bigger pot, it causes the roots to become more dense. If you start 2 or 3 seeds in a small pot, then check on them when they are bigger, you can chose the strongest, healthiest one or separate and save them all, potting them into larger containers.

Don't forget to harden off before planting in the garden! This means putting them outside in a shady spot and slowly introducing them to full sunlight and breezes for a few days before planting them outside.

Linda

"So, if as you say, 2 or 3 plants in the same whole will still produce
the equivalent of 1 plant in that hole why not simply cut it back to one
plant? What is gained by leaving them all vs. what is potentially lost
due to over-crowding and stress?"

As I said, what is gained is that if one plant dies, you still have another filling the space. When transplanted into an 18-inch space, one dies, and you have an 18-inch gap. For small gardens, where you may just have a few slots for plants, that's a big deal.

You don't lose anything from overcrowding and stress compared to what you'd have with a single plant. That was my point.

The mistake here is thinking that both plants have to each do as well as one would by itself. Yes, there is competition. You don't have space for two full sized plants in that one slot. Don't worry. You won't get two full sized plants. You'll get the equivalent of one, which is what you wanted in the first place.

Also trying to separate once the roots get seriously tangled is possible, but ends up being a major stressor to both plants. Yes, it's probably too late to separate them.

"I still don't understand why, since they don't get transplanted if they're started from seed in the garden."

Tomato plants do not get "started from seed in the garden". They are planted as plants.

Plus certain types of plants - those with woody stem epidermis - have two different types of roots. Unscientifically they are called water roots in that their focus is to supply water to the plants and feeder roots that work to supply nutrients from the soil. They have different shapes, sizes, consistency, and develop from different parts of the stem.

IF the tomato had been direct seeded it would eventually develop both types of roots but it would require several additional weeks to begin production so transplanting them one time as young seedlings eliminates that delay.

Initial seedling growth is water roots, the thick tap root only. The act of transplanting them triggers a response in the plant to develop its nest of fine, fibrous feeder roots. When you buy tomato, pepper, eggplant and a few other transplant from a nursery they have always been transplanted at least once and usually twice for the purpose of developing those roots.

There are numerous discussions here about all this if you want to read through them and see photos of the different stages of root development as well as comparison photos of the root development in transplanted vs. non-transplanted plants.

This is only one of the reasons to avoid starting seeds in large containers.

Dave