Yes, it’s true that this happens as I mentioned before, but sometimes when it’s more than just two stems fused, the bunching effect and competition for the nutrients and water funneling through just one area with passages that are sometimes constricted due to the fusing as well as the multiple overlapping leaves competing for the sunlight can create limiting effect that actually is detrimental to growth.
(...bah! I just woke up and I’m typing this without filtering from brain-speak.
Basically because they are constricted and bunched together, the “vessels” in the stems might not be able to transport nutrients and water correctly. And because they are multiple stems stuck together, there are more than usual number of leaves growing together in same space that shade each other as well as blocking airflow — if fungal issues are a problem in your garden.)
I was speaking from experience above — I had one seedling that seemed to fuse together and terminate in a gigantic mega bloom (kind of like a sunflower) — it Stopped growing and tried and successfully fruited on that single fused/fasciated blossom (actually I helped it by buzzing with an electric toothbrush), but nothing else grew — no Side shoots, leaves, until the fruit had ripened and was removed, and the fused section pruned down to lower healthy stem. Then it grew rapidly into a normal plant. This was indoors with less pest and disease pressure to attack the weakened plant.
I have also had excessively fasciated blooms on non-competition (not genetically selected for biggest tomato competition) plants which did its best to mature the fruit that set on a megablossom, but the effort took away from the rest of the plant and it rapidly succumbed to fungal issues that were plaguing the garden.
On the other hand, I have grown seeds obtained from competition lines and they do grow gigantic fused fruits without side effects. I think being selected for this trait over generations, the weaker defective performers are culled out of the line.
IMHO, however, it IS always fun to see what happens when anomalies occur, so if you are going to let these grow, please keep us posted on their progress. I’d love to find out how they will grow. (You are definitely not seeing the symptoms of Aster Yellows, then? If you are, it would be prudent to at least isolate them from the rest.)