No, unfortunately it is not possible to share the internal memory.
If both iPhones are signed in to the same Apple ID, they can share storage space in iCloud. You can buy more space in iCloud, but this will not change the space in your iPhone. iCloud is a Backup option, and not an extension of iPhone storage space.
You can get an overview of what is taking up space on your iPhone here:
Settings → General → iPhone Storage
You can read about the difference between iCloud and iPhone Storage here:
What's the difference between device storage and iCloud storage? - Apple Support
You can see how to buy more space in iCloud here:
Upgrade to iCloud+ - Apple Support