My Fusion Drive was splited in two

My brother did the initial configuration with his apple id.

Your brother should have used your Apple ID, not his. The problem is that when it is set up with an Apple ID, the OS and anything else obtained/registered with the app store will be tied to that ID forever and is not transferable.

I can't answer your question about a split Fusion drive, but one thing that you will need to do ultimately is to have your brother try to dis-associate the Mac from his account; you will then have to erase the hard drive and start over using your own Apple ID. The other alternative is to use your brothers Apple ID and password forever.

There are command line instructions to create a Fusion Drive available on the web.

In your case with what has happened it sounds like much more damage was done the just the splitting of the Fusion Drive, for that reason I would suggest you call your local Apple Store and set up an appointment for them to look at it. Ask them if they can return this to the same condition it was in when it left the factory.

Oh! By the way next time your brother what to use your Mac only allow him to use it under the Guest account where everything he does is erased when he logs off. It will make life for you so much better that way.

Please read the warranty paperwork that came w/your computer.

You have 14 days to return the computer w/no questions asked.

You have 90 days of FREE phone tech support on top of your standard 1 year warranty unless you also purchased AppleCare which gives you an additional 2 years of coverage plus FREE phone support.

Strongly suggest that you take FULL advantage of the above before it runs out. Let Apple deal w/the problems.

If you DIY and mess up, your warranty will be VOIDED!

Allan Eckert wrote:

There are command line instructions to create a Fusion Drive available on the web.

No need for any command lines.

Before doing any of below steps, if there is anything on the Mac Mini that needs to be saved, make sure that you have a valid backup of it: Mac Basics: Time Machine backs up your Mac - Apple Support

Start your mac and boot to Internet Recovery - power ON Mac Mini and after releasing power button (before mac's chime) press and hold ALT+CMD+R: let go buttons when you see spinning globe or - if you are using WiFi connection - when on the screen appears prompt to select WiFi network.

OS X: About OS X Recovery - Apple Support

When Interent Recovery is loaded, after prompt with choosing prefered language, select Disk Utility app and choose continue.

Next, select on the Disk Utility sidbar one of the disks that should be combined as Fusion Drive: Disk Utility will recognized that there is a problem with Fusion Drive and will prompt with question if you want to fix it. Selecting "FIX" button will re-create Fusion Drive and most likely erase all data on both drives.

More info: Mac mini (Late 2012 and later), iMac (Late 2012 and later): About Fusion Drive - Apple Support - check section "Lern More - Troubleshooting".

EDIT:

Learn more

Troubleshooting

• The version of Disk Utility that comes with Fusion Drive is unique. Earlier versions of Disk Utility can't be used with a Fusion Drive.
• If your system will not start because you see a flashing question mark or another alert:

Press the Power button and hold it down to power off your system.

Press the Power button again. After you hear the startup chime, press and hold Command-Option-R to start up to the Internet version of OS X Recovery.

A globe should appear with a "Starting Internet Recovery" message. It is now OK to release Command-Option-R.

A progress bar should appear and Internet Recovery should start and ask you to select a language.

Once you select a language, click Disk Utility in the window that appears and then click Continue.

If your Fusion Drive can be fixed with Disk Utility, you'll see a Disk icon with red lettering. Click the disk icon.

A dialog box appears offering to fix the Fusion Drive (shown above). Caution: Clicking Fix will erase your Fusion Drive. If you have a Time Machine backup of your Fusion Drive Mac, see Mac 101: Time Machine for information on how to restore from a Time Machine backup once the drive has been erased.

After the process is complete, the volume will change color to black. Quit Disk Utility, click Install OS X, then click Continue.

Back up all data, then repair the Fusion Drive (see under "Troubleshooting" on the linked page.)