Do double strike creatures attack twice?
Also, would {pyreheart wolf}'s ability stack with {stormblood berserker}'s?
double strike means the creature deals damage both in the first strike and normal damage steps. stormblood and pyreheart wolf abilities stack in the sense of you want to block the beserker and the wolf is attacking also you fulfill both conditions. which in this case are basically identical so you can block the beserker with just 2 creatures.
The opponent would not have to block with 4 creatures if that is what your asking.
The wolf and the berserker would be like giving {Typhiod Rats} Deathtouch. Its redundent. It doesnt stack
they do stack they're just redudantly similar. if either loses their ability at least there is a copy of the ability still active