I'm not sure how you were planning to make adjustments to the cage, but have you considered putting the two units side by side and removing the pannels that desperate them to make one large space? Here are some examples of this:
(broken link removed)
https://www.guineapigcages.com/foru...-use-a-critter-nation-cage-for-our-guinae-pig (scroll down in this thread for pictures)
(broken link removed)
https://www.guineapigcages.com/foru...-use-a-critter-nation-cage-for-our-guinae-pig (scroll down in this thread for pictures)