Tandem axle trailers the same size of a single axle trailer will be a much smoother ride when the numbers are dialed in. Made a living pulling both (enclosed trailers). Never again.
Without knowing any of your tow vehicles GAWRs and GVWR/vehicle type and not showing any actual scaled axle loads from the tow vehicle and trailer axle loads.... all we can do is guess.
Guessing leads to speculations and far out there opinions that won't help you with your issues.
Get some scaled weight numbers/measurements before and after. Then work with those numbers.