When you’re dealing with cucumber plants it’s even too much with two plants in each grow bag:
One cucumber plant in each grow bag is a better solution.
Last year I did in fact only plant two tomato plants in each grow bag, with a good result. Here’s a picture from last year:
I’m building up my courage to try and grow tomatoes without the self-watering boxes and just grow them directly in the ground. I guess the boxes are not old school enough for me after all 😉