I did this when mine started squashing up instead of separating into different coops. I removed the nestbox from one and pushed the 'gap' up against the pop hole of the other. At night they still crammed into the one coop anyway, but during the day they used both coops. It was easy to do and it was easy to reinstate the coop later.
Eventually I got two Eglus but they cram into one of them - or half of one of them, on cold nights! For summer I want to put the Eglus back-to-back as one but I'm not sure how it will work.
There are now 4 little coops in the run, mainly to give more space at 1st and 2nd floor levels, and the chickens use all of them for their own purposes (sunbathing, nesting, chilling...). They seem very happy but it makes extra chores for me, checking and cleaning etc. in comparison with having one proper hen house. The run isn't wide enough for a big coop, but that's because the garden is narrow, so for me this is the best option.