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.
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.
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.
Keep in mind when you are assigning an email to your To Do list, if sorting by 'client', the email will be found under the primary contacts name.