...strictly speaking, but the other is very common; the 'mistake' (I'd say it still is, in formal usage, but I doubt if that status will survive for long) comes from the conflation of two perfectly correct forms of words - 'both of them' and 'the two of them'. The 'perpetrator' starts to say 'the three of them' (or some other small number), then counts in mid-phrase (after saying 'the') and says, in correction, 'Both of them'. So what s/he says is "The... [no, make that] Both of them". After a few such corrections, people start saying 'the both of them', and sooner or later it becomes standard. (It already is, in some registers.)
If in doubt, you could always paraphrase: "I hid
them both."
b
for the both of them or for both of them 
Linear Mode
