We don't know what kind of witchcraft has gone on in the past ten years or so with these two. Go back to the turn of the century, and Brad Pitt and Benicio Del Toro looked nothing alike. Look at them now, and it's like seeing two photos of the same person; one taken when they've just woken up (which would be Del Toro), and one when they've had time to groom themselves and prepare for an evening out. That, obviously, would be Pitt. This isn't intended as a slight at Del Toro's expense either; most men would be flattered to be compared to Pitt in any way at all, even if he is in his 50s now.
Pitt seems to have entered a grunge phase as he's got older, tending to wear his hair longer, and experimenting with several lengths of beard. Whether he picked up any tips directly from Del Toro, who's been in beard club for a very long time, is unknown.