They just multiply each other. Crit starts at 140% damage and goes up from there if you add more crit (along with the rate). DH is 125% damage and only the rate improves with gear.

So you do 100 damage.
Crit its 100*1.4= 140 damage.
DH its 100*1.25=125.
DH & Crit its 100*1.4*1.25= 175.

175% base DH Crit and that number just goes up as you add more crit.