Deadly Strike and Crushing Blow are attached to the weapon you use them on. DS and CB from other sources (helm, armor, etc.) they count on both weapons.
Weapon 1 has 20 CB
Weapon 2 has 20 DS
40% of CB and DS from other sources
Attacking with Weapon 1 gives 60% CB and 40% DS.
Attacking with Weapon 2 gives 40% CB and 60% DS.
There is no cap on either one, but they don't do extra for going over 100%. Deadly Strike and Critical Strike are exclusive. One is checked, then the other is checked. To check your chance for double damage, use this formula:
Double Damage % = CS + (DS/100)*(100-CS)