You could use your digital camera, with all of the settings done manually. Take a photo of the plants in each, with the same settings, and compare the brightness of the photos. At best you will see if one tank has considerably less light than the other.
Two identical tanks, except one is twice as deep as the other, will have different light intensities at the substrate by a factor of 2 to 4, depending on the type of bulb being used. But, your eyes won't be able to tell the difference unless the tanks are side by side, with both light fixtures having the same bulbs in them. Your eyes are a digital camera set on "auto".
That 2 to 4 difference will only be critical if the light intensity in the shallower tank is still just adequate for the plants you have. If it is ten times what is needed, both tanks will do well.
Two identical tanks, except one is twice as deep as the other, will have different light intensities at the substrate by a factor of 2 to 4, depending on the type of bulb being used. But, your eyes won't be able to tell the difference unless the tanks are side by side, with both light fixtures having the same bulbs in them. Your eyes are a digital camera set on "auto".
That 2 to 4 difference will only be critical if the light intensity in the shallower tank is still just adequate for the plants you have. If it is ten times what is needed, both tanks will do well.