Were the plants all the same size in the 6 pack when you bought them? If not, it is possible the 2 stunted plants had been stunted due to injury or exposure before you bought them.

Is it possible that the stunted plants were more than 1 stem in the 6 pack? The plant on the left above is clearly 1 main stem but can't see the stem of the one on the right. Sometimes multiple plants will sprout in the same cell and if the extras are not removed all of them are stunted and some develop fused stems.

Are there any signs of insects on the underside of the leaves? Both aphids and especially Psyllids can stunt plants.

There are a couple of viral diseases that can cause stunting but if that was the cause one would expect the plant to look discolored and wilty too.

And can you tell me where you are located and how long the plants have been in the ground so there is some idea of the effects of weather if any?

Lastly, when planting was there any signs of soil dwelling pest like grubs etc. that could be damaging the roots of the 2 plants?

Thanks for additional info.

Dave

As I look at your plants ( 4 in the picture) they all appear very healthy, though different in size.

Other than what has been said:

Plants are like humans in a way. They are not 100% identical. If your Romas are hybrid, it is possible that maybe they came from different fruits or source or the seeds were not true.

I have about 4 Siletz that I have started them from seed and have planted them the same time, in the same bed. Some are more vigorous than the others.

But I would just take them as they are. If they are determinant, then it should fine to grow bushy.