The rapid heal prayer has the effect of doubling your hitpoints restoration. This has the same effect as the regen bracelet. The effects of the regen bracelet and the rapid heal prayer stack. So your hitpoints restore at 2x or 4x times the restoration speed.  The third thing that has this effect in the game is the hitpoints skill cape. You get this cape when you achieve level 99 hitpoints. The hitpoints cape its effect does stack with the regen bracelet OR the rapid heal. The three items do not stack when you are using all of them at once. So your maximum restoration is 2x times 2x (4x the speed), and not 2x times 2x times 2x (so 8x). In seconds, your maximum is hitpoint per 15 seconds and not one hitpoint per 7.5 seconds.