iCloud on two devices.

HI,

I have two iOS device, iPad Air 2 and iPhone 5s and I use iCloud to back up my data on both. My question is, I have photos to be backed up on both devices but I use photostream, so both devices have the same photos. Do I need to back them up on both devices, or could I just back them up on one since they both have the same photos?

Hope that made sense 🙂.

Thanks for any help.