Hi,
Based on my knowledge, Storage Spaces doesn't support the notion of shrinking a Space that was created as a specific size (especially if it's set for fixed provisioning). And replace with 2 smaller disk is not the recommendation method.
If you really want to take the size of the storage space down, you will have to create a second, smaller space, copy your files into it, then delete the space that was too large.
Based on my knowledge, storage replica and dfs replication are not suitable in your scenario to sync data. For other build-in feature, I haven't find such one to do this.
And if you don't want to move data to the new volume by copying, maybe you could also consider try to backup all the data first and restore to another volume.
Please note, keep a backup for the data before you do anything in case of something wrong.
Best Regards,