I am planning to build a membership website where that 2 websites have 1 membership area? Is that possible if I'm going to have 2 databases?
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.