Phone number and Google+ account
Can't you just split the contact up in two? so if the account have two Google accounts it is just two contacts on your address book. 
It looks to me that you are only able to choose between more numbers, so if your contacts have multiple phone numbers you can choose between them, but emails you aren't. (even when they are added as Hangout emails and is a verified Google+ account.) 
The way i would do it is split them up in multiple contacts and then make a group chat with that person that have more then one Google account.