Here is another mango seedling with wilting leaves.

Also, there are two trees on one seed. What should I do with them?

Looks like some root rot, from poorly drained soil. Take one out and check the roots. For an ID you'd need to show photos of what the fruit looked like. Where did you get the seed? If it was from a shop bought fruit, do you know where the fruit came from (sticker on the fruit/label on box)? There's something in the order of 2000 varieties of mango, they've been in cultivation for over 4000 years.

Thanks for that. Is there anyway to stop root rot?

Drench the soil with a fungicide and give the seedlings much better drainage. Growing a mango from seed is the epitome of optimism because seedlings will not be a replica of the parent tree. Once your seedlings are a few feet tall you would be wise to graft a known variety onto the root stock. Otherwise you may wait 10 years to see your first fruit and find out that it's not worth eating.