Blazing Durandal stack

heavy blade activates any time you attack the enemy, enemy or player phase. where breath skills activate when you attack and are attacking on enemy phase only.

if you had both, heavy blade would only work on player phase where steady breath would take over exclusively on enemy phase. so not stacking.

EDIT: cant inherit breathes on cavalry units :x

Heavy Blade(=HB) works only on the units attacks regardless who initiated combat.
Steady Breath(=SB) works only if the enemy initiated combat, but procs on both your attacks and the enemies.
Cooldown reduction skills like them do not stack when both proc, however with both together you would get +2 when you initiate on your attacks and +2 on all actions when the enemy initiates.
HOWEVER cavalry units can't run SB.

While Heavy Blade takes effect regardless of Phase, it only triggers when your unit attacks. Steady Breath's effect applies only on Enemy Phase, but doesn't care whether you tank or attack.

When you have the two, then Heavy Blade procs as normal during Player Phase, but only Steady Breath procs on Enemy Phase (so you don't get +2 extra charge with both skills in Enemy Phase, just +1 extra from SB). In that sense it doesn't stack, but you can still run both skills.