I agree, it would become very difficult since it tracks your progress and specifically decides which questions to show you based on if you got them correct or not. I did not use Zuku but at least with VetPrep, you cannot reset sections, the progress is permanent. I would just suggest swapping the resources of each program with one another - VetPrep PowerPages/Lectures for the Zuku Top 20 for each species. The questions are going to be similar and sufficient for you to prep for the NAVLE.
Also, I wouldn't have had the time or motivation to go through both programs even if I wanted to, lol. Studying for this exam is intense and draining. Plus, I can't recall Zuku's guarantee, but for VetPrep if you hit 80% completion, that's their guarantee that you'll pass and if you don't, they give you another subscription for free to take the exam again. So, if you share the program and in the very OFF chance you both fail...only one gets a new free subscription.