It's funny when folks say "they're EXACTLY the same", but they're not, lol.
Just looking at the photos, the "mulches"
look different. I'm not sure what it is, but the first photo looks much more dense, which will reduce evaporation and keep the soil moist longer than the "wood chip" mulch in the second photo. Might be the cause of the "drooping". Dunno.
So, I'd take a moment and think about all the things that affect the performance of each box. For example:
- the make-up of the growing medium in each box. Is that truly identical? Maybe so....maybe not?
- same amendments, added at the same rate?
- what about sun exposure. Do they both get identical sun at the same time of day? Is one more sheltered, eg; shade of a tree, the house? Is that a shade cloth over the box in the second photo?
- watering - same amount at the same time?
Anyway, you get the idea.
All in all, I think your plants look great, especially for all this extreme heat that's been going on. Good luck and hope you have an awesome grow this year!