Manage 2 iCloud accounts without paying for 2 different storages

You can simply use Family Sharing if you have a storage plan of 200 GB or 2 TB.

After, seeing your explanation:

You can use the Apple ID which has a storage plan on your new iPhone or you can cancel the storage plan subscription on the old Apple ID and start a new one on your new Apple ID.

Each of your iCloud accounts come with a limited amount of "free" storage. Adding additional storage, as you know, will come at a cost to you. Each iCloud account is considered separately by Apple and they cannot simply be merged.

No.