1. The top of the flange should be above the level of the floor by at
least an 1/8 of an inch.

2. You can add another flange extension ring. Make sure you use a
seal ring between the flange extension rings. Some mfgs include a
"rubber" ring to seal between but you could also use part of a wax
ring. Key thing is to get a good seal.

3. Connection between.. If you can use a wood screw to hold the
flange extenders tight to the floor that would be the best but it is
also ok just to install long flange bolts.

4. I don't recommend 2 wax rings.. I have seen to many of them get
off center and end up blocking the line.

Randy