Depending on your requirements there is a second method to quickly resize and add a second volume: resize the CoreStorage Volume (instead of the whole group) and add a second CoreStorage Volume inside the existing CoreStorage Volume Group. This procedure doesn't change the partition map (in contrast with the procedure in [my other answer][1] here).

**Preparation:**  

- Backup your data.
- Detach any external drive (especially your external Time Machine backup drive) except your external boot drive.
- Boot from your external drive or a bootable installer thumb drive (Mavericks or Yosemite)/a thumb drive containing a full system (Mavericks or Yosemite).

 or 

 Restart to Internet Recovery Mode by pressing <kbd>alt</kbd> <kbd>cmd</kbd> <kbd>R</kbd> at startup.  
The prerequisites are the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated.  
On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot to a recovery netboot image which usually is loaded from an apple/akamai server.
 
 I recommend ethernet because it's more reliable. If you are restricted to WIFI and the boot process fails, just restart your Mac until you succeed booting.

**Modify CoreStorage stack:**

- Open Disk Utility and check your disks for errors. Repair them and quit Disk Utility.
- Open /Applications/Utilities/Terminal
First you should get an overview of your disks and the partition layout:

- Enter `diskutil list`: 

 Example listing:

        /dev/disk0
           #:                       TYPE NAME                    SIZE       IDENTIFIER
           0:      GUID_partition_scheme                        *500.1 GB   disk0
           1:                        EFI EFI                     209.7 MB   disk0s1
           2:                  Apple_HFS Fenchurch               80.0 GB    disk0s2
           3:                  Apple_HFS BxxxxxData              70.0 GB    disk0s3
           4:          Apple_CoreStorage                         349.0 GB   disk0s4
           5:                 Apple_Boot Recovery HD             650.0 MB   disk0s5
        /dev/disk1
           #:                       TYPE NAME                    SIZE       IDENTIFIER
           0:      GUID_partition_scheme                        *100.1 GB   disk1
           1:                        EFI EFI                     209.7 MB   disk1s1
           2:                  Apple_HFS BlinkenDonx              99.9 GB    disk1s2
        /dev/disk2
           #:                       TYPE NAME                    SIZE       IDENTIFIER
           0:                  Apple_HFS YoodenVranx            *348.6 GB   disk2
                                 Logical Volume on disk0s4

 with disk0: your internal disk, disk1: a fictional external boot drive with 100 GB containing your boot volume (BlinkenDonx) and disk2: your CoreStorage Volume Group YoodenVranx residing in disk0s4.

- Enter `diskutil cs list`  

 Example listing:

		  CoreStorage logical volume groups (1 found)
		|
		+-- Logical Volume Group A629E051-D7B0-4B8C-A803-074F62704636
			=========================================================
			Name:         YoodenVranx
			Status:       Online
			Size:         349046696192 B (349.0 GB)
			Free Space:   16777216 B (16.8 MB)
			|
			+-< Physical Volume 90C09FC0-4215-4871-901B-70E2C9C7D464
			|   ----------------------------------------------------
			|   Index:    0
			|   Disk:     disk0s2
			|   Status:   Online
			|   Size:     349046696192 B (349.0 GB)
			|
			+-> Logical Volume Family F6962E38-50E4-4458-BFE6-CF2E179352F5
				----------------------------------------------------------
				Encryption Status:       Unlocked
				Encryption Type:         None
				Conversion Status:       NoConversion
				Conversion Direction:    -none-
				Has Encrypted Extents:   No
				Fully Secure:            No
				Passphrase Required:     No
				|
				+-> Logical Volume BD36C73D-860D-4DC6-B125-AD624F448B88
					---------------------------------------------------
					Disk:                  disk2
					Status:                Online
					Size (Total):          348696696192 B (348.6 GB)
					Conversion Progress:   -none-
					Revertible:            Yes (no decryption required)
					LV Name:               YoodenVranx
					Volume Name:           YoodenVranx
					Content Hint:          Apple_HFS

- Now shrink the existing CoreStorage Volume with:

        diskutil cs resizeVolume LVUUID size

 with LVUUID: UUID of the existing Logical Volume. Example:

        diskutil cs resizeVolume BD36C73D-860D-4DC6-B125-AD624F448B88 279500m

- Add a new Logical Volume in the existing CoreStorage Volume Group with:

        diskutil cs createVolume LVGUUID type name size

 with LVGUUID: UUID of the existing Logical Volume Group, type: file system, size: percentage or size. 

 Examples:

        diskutil cs createVolume A629E051-D7B0-4B8C-A803-074F62704636 jhfs+ ElCapitan 100%

  All the available free space in the CoreStorage Volume Group will be allocated to the new Logical Volume.

        diskutil cs createVolume A629E051-D7B0-4B8C-A803-074F62704636 jhfs+ ElCapitan 50000m

  50,000 MB (50 GB) of the available free space in the CoreStorage Volume Group will be allocated to the new Logical Volume.

- Enter exit and quit Terminal.
- Open Disk Utility and check your shrunk and the new CoreStorage volume for errors.
- Quit Disk Utility.
- Reboot to your main volume.


  [1]: https://apple.stackexchange.com/a/209176/93229