One membership area for two websites

I am not sure if you just want to share the same user logins across websites or that you want to set up a third platform for the users? In the first case I recommend looking at the answers here, they may be useful: https://wordpress.org/support/topic/how-to-share-logins-and-users-between-entirely-separate-multiple-wordpress-sites/

Also https://wordpress.org/plugins/ultimate-member/ is well suited for multisite installations. I have used it once and I can recommend it.

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