1. Yes
2. Unsure
3. Your selection could be tweaked.. I got the Dual saddle with puck replacement, which allows fitting of Vixen dovetails and Losmandy D dovetails.
http://www.admaccessories.com/D_Series_Dual_Saddles.htm
You may want to mount piggyback where youll need solid clamp rings for your ED120 where you can mount a Losmandy D bar on top so you can use these to mount your ED80 on top and have the ability to move it up or down the ED120 to get the correct balance needed.
http://www.admaccessories.com/D_Seri...able_Rings.htm
You could get a set of Parallax rings for your ED120 which are solid and well made, and allow you to mount to a Losmandy D bar on bottom and top so you can use this adjustable ring set for the top scope.
This will give you a solid scope setup.
Side by Side setups are nice; but you'll be tackling balance issues and flexure unless you get it just right.
Piggyback setups are easy to balance out, but you will need to counter with the right weight on the counterweight bar.
Hope that helps.
Dont be afraid to email Anthony at ADM directly. Nice guy and very helpful. He often deal with Joseph from Parallax and will know what threads to use for the Dovetail and rings sets.