I haven't heard of a split cable before, the old days of master/slave ATA are apparently gone.
What I'd do is that I'd take the drive that has the least storage space in general, relocate the data somewhere else temporarily, format and sell it, then buy another drive with ridiculous storage, maybe a WD red or blue. Its hard to get a >decent< SATA expansion card for under 50€ which could be easily invested into bigger storage.
There are also portable hard drives you can invest into, also NAS machines, that kind of things, and it saves the hassle of relocating all your hard drives if you ever decide to do a major overhaul of your system.
Oh, another biggie is that you buy a whole new PC and make a network file server. Thanks to AMD's APU's, such solutions are cheaper than ever before!