If you purchase a PPV via the 500's channels on one of your STB's, you may view that same PPV on all the STB's on your account at no additional charge. However, you will have to go to each STB, and go through the same prompts about "Do you want to buy," etc. Just go through the motions and "purchase" it by saying "Yes." You will NOT be charged any additional money for responding in the affirmative for each of the subsequent STB's on your account. I do this all the time and have done so for years.
So, to recap: purchasing a PPV on the 500's on one STB does allow you to view the same PPV (often more than one channel is unlocked allowing you several viewing times) on all the STB's on your account, but you must go to each STB and answer in the affirmative to all the same prompts for purchasing the PPV. I hope that is clear. This also provides each STB to have its own viewing experience at the control of each person in each room, and is less cumbersome than wiring, unless you love that stuff. It can be fun.
Please be careful to "purchase" the PPV on your other STB's before the end of the 24 hour cycle--within the same cycle as the first PPV purchase. If you purchase after the 24 hour viewing end time, you will be charged again because it is considered the next day.
Your STB's will call-in with the PPV info at different times during the month, but as long as the PPV's were purchased within the same 24 hour viewing cycle, there will only be ONE PPV charge on your bill. This is in regards to the 500 channels (and other PPV such as sport, etc.) and NOT DishONLINE. I don't know if Dish charges per STB for DishONLINE, but I would like to know.
Have fun.