Apple ID Switch/Split
An operation may be needed to separate the 'siamese' twins ;-)
It is not going to be totally painless. 

Start by investigating Family Sharing - which will enable you to keep the apps/music/books etc you have purchased.

Family sharing also necessitates designating one of the accounts as 'payer', so you may still have to negotiate who pays who for what, going forwards.

However, any data within each app, including anything like game progress, web &amp; other history, contacts etc is going to end up belonging solely to one brother. The other is going to have to start from scratch.

Contacts can be downloaded as .vcf files from the iCloud web page.
Calendars can be shared between family members.
...but I think just about everything else is going to be starting over from scratch.