Are you administering subQ fluids to your CRF cats? If not, that's something to discuss with your vet, because constipation in CRF cats is generally caused or exacerbated by dehydration. Also, stool softeners like Lactulose and Miralax only work if the cat is properly hydrated, because they hold water in the intestinal tract to soften stool as it forms.
Assuming that your cats are maintaining good hydration, you should ask your vet about Miralax or Lactulose. They are both osmotic laxatives that do a very good job of softening newly forming stool (they don't, however, soften already formed, constipated stool). I prefer Miralax because it is available OTC, and it is MUCH easier to administer than Lactulose. Miralax is a tasteless, crystalline powder that mixes easily into canned food. Lactulose, OTOH, is available by prescription only and is an extremely sticky, thick, sweet syrup that cats tend to reject.
Both are dose-to-effect drugs, meaning that you start with a low dose, then gradually adjust up or down until your cat's stool is the desired consistency. Too much will produce diarrhea. Too little will result in hard stools.
They are not dehydrated and do not get sub-Qs. I take Lactulose myself. It used to be tasteless but the pharmacies around me get the minty kind. Yek. I was just reading the back of a miralax bottle and the bottle tells people who have kidney disease should not take it.
How are you evaluating your cats' hydration? Does their bloodwork demonstrate proper hydration (Total Protein in the lower half of the reference range)? Are their coats silky and smooth (well hydrated) or clumpy and coarse (dehydrated). Are their gums slimy (hydrated) or tacky (dehydrated)? Do your cats drink excessively (dehydrated)?
Unless your cats are in early stage CRF, it's likely that they are dehydrated if they are not receiving supplemental fluids, and even mild dehydration can cause constipation, esp. if fiber is being added to the diet.