iCloud storage

iCloud storage is not tied to a particular device. Its tied to an Apple ID / iCloud Account.

If you use the same Apple ID for iCloud on both devices, then both devices will have access to the same amount of iCloud storage

When you sign out of the account, it will ask you if you want to keep the photos on the device. Answer Yes.

When you sign in to the other account, it will then ask you if you want to merge the photos on the device to the new iCloud account, again answer yes.

Also signing out of the iCloud account on one of the devices, does not delete the iCloud account from existence, nor does it delete any of its contents.

You can, should you need to, continue to access the account through the iCloud.com website if required. So you should not lose any photos on either device.

How can I get the 2 devices linked to the same account?

Sign in to iCloud with the same Apple ID on both devices.

NOTE: you will be abandoning any acquired content like apps, music, books, photos taken etc., associated with the Apple ID you are not using. If needed, you can use Family Sharing to share most of that between the 2 Apple IDs.

On both devices go to Settings->Account(top of settings), and see what account is signed in to iCloud there. You can scroll to the bottom of the screen and sign out form iCloud there, and then sign in with the other account.

Details here: Sign in with your Apple ID - Apple Support

You can keep access to all purchases as long as you don't change the Apple ID being used for iTunes & App Stores. They are different services and can use different Apple IDs.