If you have cross-regional accounts, use single sign-on (SSO) to reach all of your marketplaces from a single sign-in and browser. SSO lets you access any of your accounts with any set of account credentials. Then you can use the marketplace switcher to alternate between your marketplaces.
Single sign-on is already in effect if you have linked your accounts. If they are not yet linked, visit Linking accounts and follow the instructions there.