The H&H doubles are true art in steel and wood! If you get one, you will want to have it where you can gaze on it lovingly from time to time! I've never shot a .500 H&H. But, I have shot a Westley-Richards .450 Nitro Express double rifle.
While a double rifle gives the hunter the option of a very quick second shot on dangerous animals, the heavy recoil has been known to cause the second sear to jump if the spring has weakened. I have seen this happen, and the two shots going off almost together caused quite a bruise on the fellows shoulder!
Years ago, I had a .416 Rigby built on a M1917 action. Beautiful rifle! It came with 110 rounds of Kynoch 400 grain solid bullet ammo. At the time, I had dreams of being able to go on safari.