What to do with the empty pot

use seedlings pots.. size-appropriate pots always makes things easier - not just watering. If a seedling doesn't pop you aren't tying up an entire pot of medium... it's an insignificant volume and doens't prevent you from planting a few extra if you think things have gone poorly. Always a good idea to plant a few extra beyond your needs to cover for mutations and any that fail to sprout. Easier to justify when the extra seeds are free or at least not exorbitantly priced. I'd avoid putting more than one seed in a pot. One usually dominates the other and there's no real benefit to doing it. if you can transplant it early enough that the roots aren't all entangled and you don't cause a bunch of damage to each root system, sure you can move it. Based on size of these, i doubt that's the case. Again, not something i'd recommend. I'd suggest just killing one when you see which is dominant. Anything over 10-14 days and the seed is probably inviable or rotting. Before that there's still some hope.