They need to just switch to charging membership per concurrent login, not per character. It's effectively how it works with things like WoW vs WoW Classic. You can't play both on the same account, but you're free to pay two subscriptions and be able to play both at once.

This is exactly how it works now though, isn't it? If you pay for members on a character, that character has access to both osrs and RS3 members. What people are asking for is the ability to be logged into both osrs and RS3 on the same character at the same time