Am I being dumb?

Zanderman

p00hsticks said:

Zanderman said:

If you want proper joint access - i.e. an account in both names, you'll probably need to visit branch for most banks and most accounts, savings or current. 
There might be exceptions though - a bank where you both bank already perhaps?

I suspect the OP and brother are deliberately avoiding banks where one or the other already has online accounts as this would involve sharing existing online banking passwords etc.

Whereas with a completely separate bank and a new joint account the login details can be shared with impunity between the two.. 

Not sure why you think an existing bank is a problem.
If X and Y both have separate accounts and logins at Toytown Bank and now need a joint account at Toytown Bank, they open one. 
When X logs in they see their own old account plus the new joint one. 
When Y logs in they see their own old account plus the new joint one.
Easy. 
No sharing of any existing passwords.