They weren't trying to move the door but break it. Let's say the door was sturdy enough to take hits on one side but if hit simultaneously it won't resist and instead will crack. That's the point of hitting it on both sides. If Salamence cared for Heather at all, they would try their very best to get her out regardless of her mindset. I'm not saying that like Salamence doesn't care for her, I'm just pointing it out.
xD Look at us, going all philosophical about a door.