Ok I haven't had time to test this, but what if someone used demonic flesh(which increases max health) and endure pain(does the same) at the same time would they both work together?
Definaltly yes. Whenever something say +xxx of something, it adds on to whatever is there. So if a skill does +28 dmg, you would do your normal damage as well as the extra 28. If the skill just says 28 dmg, then the target just takes 28 dmg.