Correct: (with some slight variations in meaning)
between them
between the two of them
between those two
between these two
Wrong:
between them two
In 'between them', 'them' is the object of the preposition, so must be objective case.
In 'between them two', 'two' is the object of the preposition and 'them' is incorrectly attempting to modify 'two'. (
Which two?
Them two!)
ETA: I agree with IceCream -- I have a slight preference for 'them' or 'the two of them'. While correct, 'these/those two' feels just a touch more casual or colloquial.