MultiValue is called multi-value exactly for that reason - one object, multiple values. You don't need to create two separate objects.
See this example:
import AddressBook
from AddressBook import *
ab = AddressBook.ABAddressBook.sharedAddressBook()
p = ABPerson.alloc().init()
p.setValue_forProperty_('JOE', kABFirstNameProperty)
p.setValue_forProperty_('TEST', kABLastNameProperty)
phoneNumberMultiValue = ABMultiValueCreateMutable()
ABMultiValueAdd(phoneNumberMultiValue, '555-555-1212', kABPhoneMobileLabel, None);
ABMultiValueAdd(phoneNumberMultiValue, '555-555-1213', kABPhoneWorkLabel, None);
p.setValue_forProperty_(phoneNumberMultiValue, kABPhoneProperty);
ab.addRecord_(p)
ret = ab.save()
In other words, you create one MultiValue and add different values there under different labels. This example is tested and working on 10.8.3 with python 2.7.