You definitely want both blades on. Mowers should NEVER be run without the correct and properly balanced place on the spindle/shaft. You should have no problem tackling this problem at the highest cut height and taking it slow. Make sure that your blades are sharp, your tires are properly inflated, and your mower is running at full RPM. Most of the time, even with $10,000 commercial machines and taking it slow, a second or third pass is needed. While it is most taking on the power of the engine, a 2-1 or high lift blade will do the most good for this type of cutting because you want to get those clippings out from under the deck so that the uncut, tall grass can be cut with each subsequent pass. Good Luck!!