I assumed that they would share the space.
I don't know how to get them to share.
Yes, that's the same as putting your car and your bike in your garage.
You share iCloud+ plan for your Mac and iPhone.
See the following Apple articles:
What is iCloud+? -> What is iCloud+? – Apple Support (UK)
Check your iCloud storage online or on any device
Check your iCloud storage on any device – Apple Support (UK)
What's in Others in your iCloud storage
What's in Others in your iCloud storage - Apple Support
Manage your iCloud storage -> Manage your iCloud storage - Apple Support
Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support
See also: iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support
If you have upgraded iCloud storage, once your request for iCloud+ has been activated, you should receive an invoice in your email box.
Then check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.
If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.
Use the same Apple ID.