If you associate a single email address to more than one contact, you will have to select one of those contacts to be the primary owner.
Choosing the primary owner
Email communication with that address will automatically be added to the timeline of the primary owner if email sharing is turned on.
When you are just adding the email to a secondary contact for the first time, you will be prompted to choose who you would like as the primary owner.
Changing the primary owner
If you would like to change the primary owner, simply go into one of the contact's Details tab, scroll down to Email Sharing and select Edit.
You will then be able to change the primary owner allowing emails to flow into their timeline automatically.
Changing the primary owner of an email address does not change timeline histories. Emails previously added to one timeline will not be moved simply because the primary owner of the email changes; only new emails going forward will attach to the new owner's timeline.
Adding an outgoing email to multiple contact timelines
When you send an email to an email address linked to multiple contacts, you can decide which timeline(s) that email will be recorded on.
When entering the email address, you'll see the option to add the email to one or more timelines. By clicking the link, you will be able to decide which timeline(s) that email lives on.