You have to insert the respective contact several times, for each email address.
If you insert the contact by hand (i.e. by writing the name of the contact in the To/Cc/Bcc box), if the respective contact has more addresses, the list of addresses will be displayed and you can choose the one you want. For the second one, start writing the name again and choose a different address from the drop-down list. (In the image below, the respective contact has 5 addresses, so if I want to send the message to, say, 3 of the 5, I start writing the name once and choose one the emails, then again and choose a second email, then again and choose the third email.
Else, if you insert the contact by clicking on the To, search for the contact in the box and then select each of the emails listed for that respective contact and then click Select.