I got most of my information from a friend who is an environmental engineer - he spends most of his life designing cleanup systems that require many miles of plumbing/extraction chambers etc and intricate measuring devices.
When we researched and purchased our systems he even went as far as calculating the cross sectional area of the tubes to see if they equal the supply line (1.5") cross area (which ours did).
I don't want this to get arguementative, since I hate nothing worse that threads going that way. However, just how would the backpressure increase by running MY panels in series, I'm not reducing my pipe diameter anywhere along the route (except the return valve, but that is standard for all systems, so really, that's where my increase in backpressure would come from).
I'd have to say in your setup's case, you've obviously proven that the panels are restriciting your flow - thus the 6gpm vs 12gpm output (and likely creating backpressure) - maybe the cross sectional area of the channels in your panels do NOT equal the cross sectional area of your supply piping, has anyone calculated that at all? If that's the case, then yes, run them in parallel to keep the same flow rate as if they were not in the picture, however, if one panel has the same cross sectional area it should not reduce your flow rate.
if I were to parallel my panels I'd be efectively increasing my pipe diameter and possibly incur the issues that can potentially go unnoticed as I mentioned in my earlier post, running my panels in series allows me to catch anything almost immedialtely...at worst after a backwash cycle.
Obviously, differences in equipment and manufacturers have different outcomes, and if sungrabber recommends it, tehy're probably right - they designed it, our panel manufacturer obviously has different panels.