Does a UK Seller still need separate email for a USA account?

Anyone know if UK sellers still need a separate email and display name if they open a USA account? Or any other account outwith the EU? I know there is “single sign on” which means you can access all your linked accounts once you sign in - but do you still need a different email to start with? And does your display name still need to be different? Also - is it true that accounts can be merged so that one subscription fee is paid? Thanks! : )