First, insert a refill between the existing elastic band and cover. Then, place the second notebook next to the first. Open both notebooks in the middle and grip those middle pages together. Slide the connecting band through those pages.
Using an additional elastic band to attach the first and second refill outside of the notebook, then insert them into the notebook afterwards. The back cover of the first refill and the front cover of the second becomes the middle of the notebook with this arrangement. This provides a more balanced look to your notebook if you only have two refills. For the third refill, open it in the middle and slide into the existing elastic band of the cover.