I think it is because u are using 1 vst on 2 tracks. Don’t really know how this works in Renoise as i haven’t tried it, but i have used more programs that have problems with this kind of things.U could try loading the same vst again to double it, but as dsk is sythedit, u may be better of just sampling the sounds u want to double up.
edit: i don’t know why u are trying to double this sound but if you want to have paralell fx you can also just use a send device(if this is not loud enough for what u want to do then maximizer/compressor can fix this)