API makes a phosphate test kit. Cutting one light will only limit the spread. Most low to mid level LEDs have poor spread the further off center you go (like after 6 inches), especially when resting on the tank's rim . You'll have some plants getting blasted still while others are not receiving enough. You'll get better spread if you raise just one of them and center it over the tank, but your PAR levels will drop drastically. Best to use a dimmer for both lights to get a good spread but with less intensity. You would need to beef up the power supply if you went this route because you are using one power supply to power both lights. Still can be done easily and inexpensively. I understand if you don't want to bother with stripping wires and screwing them into the dimmer. Not everyone's cup of tea. That leaves one last option. Use of a screen of sorts. I have used some black crochet mesh between the light and tank with success. Just don't attach to light itself or it could very well melt.
As for dosing, if nitrates are that high, you are wasting your KNO3. Doesn't matter what calculators say to measure out... if you don't have the plant mass to utilize it, it will feed algae. EI is based on heavy plant mass along with high light and CO2 levels. You have two of those things. Changes need to be made in your particular case. Same with everyone. Nobody really follows the EI to the tee.