Maybe you could give us a little more information about your system. I am an old codger, and to me, eOS should be on an antique CDC-ETA10 supercomputer. That eOS probably did not even have a boot loader as we know it. In general, you would install the eOS first, saving space for Sparky, and when you install Sparky it should be considerate of what is already on your disk. If you need something more specific, the installer has a manual disk partitioning option with a button to select advanced formating options. You need 30 GB for the system install "/" partition, and additional partitions for swap, and home, as you desire. Then hopefully Sparky will correctly set up the Grub boot loader for both O.S. If not, you will need to look up how to modify Grub, so that your Dual boot can work. I suppose another option would be leave eOS alone, and install a bootable image of Sparky on a USB stick, or external USB drive.