Would I be able to fit another plant in my 80x80x1...

Hello @JR420 ! It depends from how good grower you are. That size of tend can host a fully developed auto plant if you grow it at its full potential or 2 medium size. I would never put more than 2 unless i was growing autos like low rider. These autos were really small...Happy growing ! 👊

Yes you would be able to put many plants in that space that's a big space assuming that's 80"x80"x180" you could fit 8 plants in there easy ! But if your new to growing I would keep the amount of plants down . If your only doing 1 right now add a few it wont hurt but don't go overboard as many plants can be overwhelming and lots and lots of work , if your up for the challenge then fill her up and get that sea of green going ! But at the end of the day you can fit way way way more then just 1 plant in that space . You gotta remember plants can be trained to grow how you want them to :) . I hope I've helped abit !