Mine do that. its often age related. if the leaves got splashed and or crushed [certainly in the case of mine]. greenfly damage etc. just cut the affected part off or remove the whole leaf.

I do wonder if they need a higher humidity compared to where I have them.

Dry air or drafts can cause that.

Thank you for telling me this, I was quite worried that it would be some kind of fungi since it had spread to a little one that I only planted about five months ago. ¿I assume spraying them with water every so often will help?

Over watering can cause leaves to brown & wilt. Allow plant to dry out between waterings. Spider plants need very little water.

Spraying with water will help to increase humidity. It doesn't like being in a place with dry air, but the soil should not be allowed to become too wet.

hello quick update: I tried spraying them with water and cutting off the affected areas but it just kept spreading and now they both have barely any healthy leaves left, any advice?

Maybe try repotting it. Put good drainage in the bottom of the pot. Wash all the old compost off and replant in fresh. Remove any unhealthy roots.

Thanks for the tip! I'll try it out as soon as possible, hopefully they wont die.

Good Luck !

All we can do is guess what the problem could be. Be sure it's not near a heat source - radiator, space heater, fire place. It must be something drastic. These plants are troopers and tough to kill.