Caveat: this has probably changed since BC, when i actually cared about proc mechanics
Weapon procs (like venomstrike) are proc-per-minute (ppm) if i remember correctly and to flesh out what you said, that means that if something like fiery has 6 ppm, if you auto attack for a minute, itll trigger six times.
WoW averages this to proc per hit, so a fast dagger and a slow 2hand axe will have the same ppm if theyre just auto attacking.
But!
Lets say youre a warrior with an axe that has a 3.6 sec swing timer. You get 16.6 swings per minute, which is roughly one proc every 3 swings.
This doesnt just count for auto attacks. Any attack can trigger a proc. And youll get one roughly every 3 attacks. So if your warrior is, say, spamming slam on the GCD youll end up getting alot more than 6 procs in 60 seconds.
So if youre a class with an off hand specific attack (fury warriors, enhance shaman, etc) then yes. Using two proc weapons will significantly increase the chance of getting procs, because each weapon has a separate chance to proc and the off hand attack is increasing the number of attacks the offhand is making per minute.
If you dont have a specific off hand attack, youll get whatever the ppm is on the offhands auto attacks.
All this is to say, brb farming Nightreaver