Note that this is NOT a good idea to do with a x22 box. x22 requires phone line hookup and it's only a matter of time before they catch you using CallerID.
We're going to assume that this is your vacation cabin and NOT a separate family residence or such. Vacation cabin is an 'iffy' gray area for doing this - generally considered similar to RV installs. A separate residence means you're trying to 'stack' the accounts, and that's theft for sure.
So, if it's a seldom-used unit for the same family, I have no moral objection to doing this. Buy a 301 or 311 and activate it as the 3rd box on your account.
Finally, you can run into hardware damage situations when switching boxes between DishPro and Legacy. It can be done safely, but is non-trivial.