Why do a non-sequential conversion?
 

would you just do it for the same reason as getting one big turbo? to have 2 bigger turbos rather than 1 big and 1 small one? or do you use the same turbos?

The stock turbo are both the same size! You go non-seq to reduce the complexity of the controls and reduce the posibility of control failures.
If the conversion is done correctly, you will gain mid and high end power, but will lose low end responce. But then if you mod the ports and some other parts, you will gain back the low end.