It's all a string of possibilities. Costs/benefit. Do you want to be the most sure you can of the best survival of a single plant? Kill the other one in place. Do not touch the roots, don't rip them apart, your possibilities of damage/stunt/death are infinite at this moment. The second you reach to touch.
But if you simply kill one of them, the other will continue on as if nothing happened and will never compete with somebody else's roots and try to get strangled by each other because that's what they do.
Yes cooperate. Sometimes. Yes, there are various fungus that are wonderful and we do the best we can to get a cooperative environment. But these things are competitive. They only live a season. They have evolved to kill everything they can around them simply because it's what they do.
So what's your goal, how much did that seed cost or what was the effort and how sad will you be when in 3 weeks you decide to just kill them all and start over because they're stunted. Or how much time will you waste past that?
Obviously you can tell I'm the type to kill and move on.