I'd use either one in place of debit cards, just for fraud liability.
As for maxing the rewards, just do some math on what's better for your normal purchases (gas, groceries, and entertainment being the big ones probably.) Someone more knowing than I about the details of these two cards can probably answer that as well.
I have the Zync card and I do not have the Chase sapphire, however I think I could make a few points that could help. First depending on what you will be using your points for, you should ramp up points on both cards evenly. If you redeem for travel mostly--use the Sapphire as your everyday card especially at the places you earn double points. If you use points for gift cards or other merchandise, then use the Zync primary and capitalize on the double points with the packs you have. Remember the packs on Zync are only good for $5000 worth of double points so once you spend that much in that category move on to the Sapphire. Second, you can use both to its limits and if you wish to redeem Zync points for travel you can upgrade to another card later that will turn your MRE points into full MR points which are very valuable especially in the travel arena. Good luck they are great card both!