The 2 Panels are designed to run in series (by manufacturer) utilizing a standard pool pump, there are many smaller variances such as pump size, GPH , but that is the basic starting point.
It has something to do with the cross sectional area of all the small channels in the panel adding up to the cross sectional area of your input/output pipes.
Running in parallel is like taking your 1.5" pipe, changing it part way through to a 2" pipe, then back to a 1.5" pipe - this can cause inefficiencies such as stagnant or "slow" areas on the panels where there is slightly more resisitance in some of the small channels due to production imperfections (water takes the path of least resistance).
If a portion of your panels malfunctions (plugs up) you will see that immediately on your pressure gauge if they're in series, however, the only way to tell when in parallel is to physically touch the panels and feel for hot channels since your 1.5" outlet is going to dictate your backpressure in that setup scenario.
BTU should be the samel whether you run in parallel or in series since your surface area exposed to the sun is the same thus collecting the identical amount of energy) and your pump is pushing identical GPH and heat transfer to water at any temperature (outside of steam) is constant.

I'm no expert, but have had numerous discussions on this topic with a friend of mine who is prior to both him and me installing these systems on our pools.

Bottom line is to do what the manufacturer recommends since they went through all the design work and testing prior to bringing the product to market.