models-account.js

export default Model.extend({
  users:        hasMany( 'user', { inverse: 'company' } ),
  usersBilling: hasMany( 'user', { inverse: 'billingCompany' } ),
});

models-user.js

export default Model.extend({
  company:        belongsTo( 'account' ),
  billingCompany: belongsTo( 'account' )
});

response

// result (when serializers-user's `include` is commented out)
{
  user: {
    id: "1",
    billingCompanyId: "4",
    companyId: "1"
  }
}

// result (when serializers-user's `include` exists)
{
  user: {
    id: "1",
    billingCompanyId: "4",
    account: { name: 'State of Maine' }
  }
}

// expected
{
  user: {
    id: "1",
    billingCompany: { name: 'Washington County' },
    company: { name: 'State of Maine' }
  }
}

scenarios-default.js

const accounts = [
  server.create( 'account', { name: 'State of Maine' } ),
  server.create( 'account', { name: 'Aroostook County' } ),
  server.create( 'account', { name: 'Cumberland County' } ),
  server.create( 'account', { name: 'Washington County' } ),
  server.create( 'account', { name: 'Androscoggin County' } )
];

server.create('user', {
  company:        accounts[0],
  billingCompany: accounts[3]
});

serializers-user.js

export default ApplicationSerializer.extend({
  include: [ 'company', 'billingCompany' ]
});