Unless both iPads are set up to use iCloud Photo Library (beta), then 'all' photos will not sync between them.
If you are using Photo Stream on both, then you'll see the last 1000 photos on both. (But only those taken since the iPad Air 2 had Photo Stream turned on will appear on that iPad. It's not a sync service, but a quick way to see recent photos on all your devices, and save them to those devices if desired.
You can try iCloud Photo Library (beta), but you need to keep two things in mind.
One; it is a beta. There may still be bugs. One I've noticed is that I need to have the Photos app open for a while on both the uploading and the downloading device (not necessarily at the same time). The background syncing seems to be slow and unpredictable.
Two; iCloud Photo Library uses up your iCloud storage limit. It won't take long before you exceed the 5 GB of free storage and face the choice of paying for more (starting at 99 cents a month for 20 GB), or deleting photos from it constantly.
There are other solutions.
If you don't need to share everything, you can use iCloud Photo Sharing. You have to create and manage these albums manually, but they will be shared with all your iOS devices (with the same iCloud account), and do not count against your iCloud Storage limit. There is a limit on how many photos you can store there, but it's a fairly permissive limit.
Store your photos on a third party photo sharing service like Flickr or DropBox. These offer a variety of tools and methods for uploading photos. Research and choose carefully. Once your invested it will be annoying to have to do it all over again for a different service.
It's a good idea to do this in addition anyway. Don't trust any one service to keep all your important photos. Anything important should be saved in at least two physically separate locations.
I am currently using iCloud Photo Library (beta). I also manually upload what I consider my important photos to Flickr. I generally take the time to do this about once a week.
