One user access multiple accounts

We are a reseller so do not have admin/root access.

I have some users who have more than one account.
They have different domains etc on different accounts for different websites.

The users have the same email addresses for each account is it possible they can access the other accounts from within thiers.

So if they have an account with domain1.com another with domain3.com and another with domain3.com
Is there a way they can access the other accounts while the are logged in?

Just like we can as a reseller account from the top menu?

TIA