It looks scarier than it is, but you’ll get it within minutes. First go to Settings → CoreElec → Services. Make sure SSH is enabled. Then go to Settings → System Information. Note the IP address.
Open up putty, type that IP address and click “Open”. That’s it, you’ve connected to CoreElec vs SSH! The default username password is root / coreelec
Now you can just type “ceemmc -x”, and select option1 (dual boot) when it comes up. Press “Y” at the prompts that show up, and done. You’ve successfully loaded your OS into emmc and can remove the slower SD card.