You can run both ZD in redundancy mode, however they both need to be on the same licence count. So they must both be on 25. You will either have to buy a 25 licence upgrade or if conditions apply Ruckus will upgrade your backup ZD for free. Maybe contact support and ask.
I've contacted the support, found out that I already have WatchDog Redundant Controller Support for ZD1100 One Year Support and my second device is eligible for additional licenses. I had to register the product and install the licenses.
Moreover, I don't have to configure the redundancy if I don't want to. So all is solved, on chat.