2 Apple IDs in one device

If you purchase extra iCloud storage using the iCloud ID, you should be able to backup all the data on the phone, including app data for apps downloaded using the App Store/iTunes ID.

Back up – What does iCloud back up

If you decide to switch to using one ID, as suggested above, you can try setting up Family Sharing between the IDs if you want to be able to share purchases made with the IDs.

Family Sharing.         

Family Sharing - Set up.       

You need to buy the extra iCloud storage using the iCloud ID. You can tell what apps were bought using which ID by checking the Purchases History of each ID. You can consider setting up Family Sharing using the links above to share purchases with the IDs.

Purchase History Review.  

Why do you feel you need to have two Apple ID's? You are able to use the same one for all of this.

If you do decide to stick to use one Apple ID, you should remember that apps must be downloaded and updated on the same Apple ID

For example)

You purchased and downloaded Shazam on Apple ID 1 but you recently changed your Apple ID, you won't be able to update Shazam on Apple ID 2 so you'd need to uninstall it and reinstall it using Apple ID 2

To get more information about purchasing more storage for iCloud go here.