Thanks for reaching out! We regrettably don’t have a universal part that would connect multiple BILLY units together. The best option we can recommend would be to drill metal brackets into the backs of multiple units to connect them, but doing so would void the product’s warranty.
You can also put the BILLY bookcases where you want them in your home and drill them into the wall using the wall anchors you mentioned. This way, they would stay together without actually being connected.
We hope this information helps.
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