Dying yellow leaves on one out of two plants under same conditions?

You have a few problems going on the yellow from the bottom up older leafs dying off first look like a nitrogen deficiency. The yellow and brown spots on leaf is a manganese deficiency. I’ll flush my medium to make sure I’m not locking out any nutrients. Then I’ll start with a light dose of nutrients. Also most important of all because it happened to me . Is to check or re calibrate your ph pen . My pen went bad given me similar problems. You can check out my moxie to read what I did to fix my issue . Hope that helps good luck to you ✌️🏾

and indeed he needs some nutes

you should pH your water since tap water is 7-9 pH and ideal is 5,5-6,5

Looks like they might need some cal mag. Try grabbing a bottle and I believe they will bounce back! Cheers and happy growing!