Firstly each iPad will never allow you to store 16GB in total because the OS takes up some space on the storage. There is no way to avoid that. Some iOS versions are a bit more space efficient, but you never get access to all of it.
If you have the App store setup to download all purchases each device will use space for that, turn off that option in settings or delete any apps you do not want on the device in question.
If you use iCloud to sync documents & photos you will also consume space. Creating a new Apple ID may help, however you end up with data in 2 different places and can easily get into a mess. It may make more sense to decide what data is actually important to your daily usage & remove any that is not important. For example you may only want all your photos to be on one device.
If you have many GB's of data syncing from iCloud you will have issues, iCloud has very little control over what is copied to the devices, so expecting iCloud to store too much data can lead to issues like this.
I'd suggest you look a 'Settings > General > Usage > Manage Storage' to see where space is used & then ask for help to clean up those areas of the system if you need it.