If I do need to use a backup, it won't need another NAND chip, it will be another UBI/UBIFS partition. But I would like as simple as possible if no backup is a common practice.
The A/B partitioning is very common and I would argue, it is the simplest way if you want you OTA Update to be safe in regards of any errors (user unplugging etc.) that may happen during write / switching to new software. If you can spare the extra partitions on your NAND, my advice would be to do it exactly that way, and use a common tool for that (i.e. swupate, rauc, ... ).
