you will always have regitry problems with 2 burning programs installed. remove both of them an use a registry cleaner, restart and reinstall roxio. that should fix the problem.. if you need a cleaner let me know. i use an old version of jv16.
I'm using both Nero and Alcohol 120% on the same pc without problems.
You are saying you're trying to burn 2 DVD's at once? No wonder your sessions crash, your buffer would be overloaded massively while doing that.
1 session at once is the only way to go with burning...