Usually only one viewer/receiver is allowed per slingbox at any given time (I'm not sure if that's a limitation of the tech or if sling intentionally limits what the boxes could otherwise do). It seems unlikely that one sling adapter/922 could feed a signal to two receivers at once. But, if the two extra tvs don't need signals at the same time, you could be in business.
This also means that Dish will be able to feed 4HD tvs with two 722/722k receivers with sling adapters or 922 receivers, and two sling receivers. The $17 fee on the second duo receiver is not looking so bad anymore.
Hopefully this is well implemented and they decide not to add additional made up fees.