I am a professional garden and landscape maintenance service provider and use strimmers on a daily basis. I have used quite a few over the years but usually larger ones and i can honestly say that the performance of every one i have used has been much better with two lines. You are essentially getting twice the number of "cuts" per minute, there is more air flow to lift the grass you are cutting and the tool is more balanced therefore vibrating less.

I have a Pellenc battery powered strimmer which is more comparable to the one you are using and that too performs much better with two lines.

Stuart