Charles,
Here is a link that has some good discussion of tandem GMAW. The data your looking for on twin wire GMAW is about half way down the lengthy test.
The article asks good hard questions about process control, the correct power suppies for specific conditions and even some working data.
The author is a bit gruff but his info may be right up your alley
http://www.weldreality.com/equipment_evaluation.htm