@Seamus these are called polyembryonic seedlings. One will be a normal plant with genes from both mom and dad and the other will be an exact genetic duplicate of mom usually. If from feminized seeds one will be a mix of the two mothers or varying mixes of the one mom if self pollinated where as the other will be an almost exact genetic duplicate of mom.
The main embryo (or I should say the embryo resulting from the pollination) will be more vigorous and shouldn’t have any issues. The other embryo that’s a copy of mom will usually have less vigor and can sometimes mutate and have other issues that are often the result of severe inbreeding and compound feminization.
You can separate them if you want and grow them separately or you can just cull the tiny one. Up to you.
You don’t want to really grow them together like that though because if the small one survives and isn’t out competed by the larger one the roots will bind together and you’ll have issues with competition for nutrients and if one gets sick with a root issue it will give it to the other.
I personally usually separate polyembryonic seedlings before transplanting. I like to keep them just in case one is a super one off amazing plant but generally the runty ones don’t make it and will die off in a few weeks. I’ve had some make it and had decent results but nothing too amazing yet.