No, but you can plug a DishPro Dual into the "IN" plug on a DPP Twin. The switch inside the Twin LNB is designed for this, and if you're only feeding one or two receivers (even dual-tuner boxes), you don't need another multiswitch.
If you plug one Twin into the back side of another Twin, one or both will burn out.
Yes, if you run them through an external switch such as a DP34 or DP21. Haven't seen if anyone's tried with a DPP33.
See the bottom of http://www.dishuser.org/addadish.php.