"But I can go down to Fry's and buy a 2 TB drive for $69!".
I'll have to remember that one
You can't really 'split' catalogs. A IDB catalog is made up of several tablespaces. You can have Tablespace extension files if you outgrow a Tablespace, but this isn't really 'splitting' a catalog
Similarly, all details about a backup are written into Data Catalog Binary Files (DCBF). You can add more DCBF directories if needed, but, there is no way to look at one or another in terms of splitting the Catalog