Can dual hard drive kit be used with a data doubler kit?

You only have three SATA ports in your system (older models only have two). There is no way to split the SATA port (expanding beyond what you have).

Currently you have two in use (HD & Optical drive). Apple added a third port as people wanted to use a SSD for a boot drive (to speed up the boot up). Later on Apple introduced the Fusion Drive service to make managing the storage space easier as one unitized volume and allowing the OS to place the files on the best storage area.

If you want to hold on to your HD & Optical drive then you have the third port to connect your SSD to using the special Apple SSD bracket ($$) or just some double stick foam tape (the SSD is not very heavy). If you add in a second HD I do recommend the bracket as the heat dries out the tape.

If you want to add in a second SSD (or have an older iMac) then you can use the OWC (Data Doubler) or iFixIt Optical drive bay carrier replacing your optical drive.