Best way to convert two single WordPress sites into Multisite network with shared user base

To me the best way would be to:

  1. Leave the ecommerce site intact since it’s where all the important content exist.
  2. Enable multisite on ecommerce site. See official documentation:
  3. Export everything on the private site (Tools – Export – All content)
  4. Import it on a new site created on the new network.