Owh I think I know why.Example:
Healing meat w/o card=100 hp
with 2 HP Card gives(200%)=300 hp
while 1 HP Card and 1 SR Card(150%) gives=250 hp actually you wrong at this.It will never gives 150%
So,1 HP Card=200 hp and 50% of the rest(200 hp) will gives another 100 hp=300 hp.
The answer is 'using 2 HP or 1 HP+1 SR gives same amount of healing'.