3 answers
A solution starts in Pilot with Spring 15 which is pretty much now depending on what instance you are on
https://success.salesforce.com/ideaView?id=08730000000BrBSAA0
Otherwise you can look at Contact Roles as well
https://help.salesforce.com/HTViewHelpDoc?id=contactroles.htm
And there is always creating a Lookup(Account) field called something like "Secondary Account" to the Contact object
