The following links are for test purposes.
One Password
Both pages below are protected by the password BobRoss. Entering that password on page 1 allows access to page 2 and vice versa.
+ Simple for end users
+ Built in functionality and requires no special software
– May become complex if there are many pages that need the password updating.
– Members could share the password without paying membership
Password page – enter a password to see the content
Second password page – to check if you need to re-enter the same password to see a different page
Plugin System
MemberPress – paid for solution that manages membership of a site and billing.
Simple Membership – a free plugin that lets people register for the site. Accounts can be set to expire (or not).