Sometimes with these aftermarket rotors judder is caused by taking away too much of the top surface contact area and/or having the (sharks fins) too far apart.
If the juddering is only at a certain speed , say not slow but medium pace it may be harmonics, but as you said it's both wheels i'd check your pads 1st and make sure the disc isn't a tad smaller than it should be and wearing those pads a groove.
Although prob not harmonics, this can give a juddering feel when a caliper mount is not faced properly and vibes turn into bigger vibes that are then felt in the frame, but as i said as it's both wheels it most likely bad designed discs.
Also check they are both rotors are straight and pads are bitting evenly.
Voodoo Erzulie 19.5" bit too big looking for 18.5" steel Voodoo
Lots of memory bikes !
That is what I mean, if you think I meant something different....well,that is in you head , not mine !
Just think of all the people that would agree with each other, but can't because they don't speak the same language