Even if the newts are not immediately toxic to each other, there are other reasons to keep them separated. Over the long run, their behavior will be different, and the presence of another species (significantly larger) will cause stress. Also, most people who consider mixing species want to do it in a 10-gallon tank, which is too small to allow the newts to avoid each other. I won't say that mixing two species is always absolutely wrong, but it's never something I'd recommend.