I guess start by using them together. If XCHT is wrong for the dog, the azotemia could briefly worsen, but Rehmannia will prevent that from being too significant. So, follow up in about ten to fourteen days to ensure no significant worsening.
If you don't see worsening, then significantly reduce the Rehmannia and continue the XCHT. At that point, in another ten days or so, you should see improvement. If you do, you're clear to stop the Rehmannia for a bit. Note, though, that once improvements stop accruing on the XCHT, you'll need to switch back to Rehmannia to 'finish the case'
Hope that makes sense!