Do these two things stack? So have I twice the chance to injure a downed player with this combo?

Yes. Please note that the two cards are not the same:

"Smash 'em": after successfully tackling a standing opposing player […]

"Violence": after a standing player becomes downed […]

So, let's say you successfully tackle an opposing player: you roll twice, one for the "Smash" and one for "Violence". Let's say you unsuccesfully tackle an opposing player and your player is downed: this triggers the "Violence" on your player. And clearly, a "miss" result doesn't trigger either of the conditions.

Thanks, that was exactly what i meant. My intension was the blocking thing.