[Huawei Ascend Mate 2] xordos:
This guide is show you how to install second OS in SDCard, so you can have two complete separate OS (each have their own /system, /data and /cache), this will be useful if you want to try out a new ROM or a new version, like CM or latest Stock build etc. I have pre-build 2 ROMs in this guide. and list the instructions of how to do it in post #2.

The whole method is just an implementation of [MENTION=6367467]SHM[/MENTION] (Modding.MyMind) guide that inside the post here, so all the credit goes to him:

http://xdaforums.com/showpost.php?p=62460895&postcount=325

Highlight of the steps:

1. Partition a SDCard (32GB or bigger) to have 3 ext4 partitions after regular FAT32 partition

2. Flash two customized flashable zip in twrp so that:

boot image flashed to /dev/block/mmcblk0p18 (regular boot partition)

system image flashed to /dev/block/mmcblk1p2 (sdcard)

The boot image contains the fstab file, it has been updated to use partitions on sdcard as the /system, /data and /cache for the 2nd OS.

And of course, the flashable zip has updater script changed so that, it will flash they system image to correct partition on SDCard.

Note / Limitation:

1) After boot to 2nd OS on the SDCard, theoretically, while you are in Android OS, everything should work. The speed/performance may differ depends on the SDCard read/write speed.

2) TWRP, except flash the zip to install the customized system image to SDCard, all other function will not work for the 2nd OS, The backup/restore function is still working on the primary OS /system, /data etc.

3) Boot partition (/dev/block/mmcblk0p18), this partition will decide which OS it will boot, and the boot image should corresponded to your OS system partition. So if you flashed above boot image for 2nd OS, it will find /system on SDCard and boot it. And if you flashed (or restore) your primary OS boot image, then it will boot to your primary OS /system.

Things you need before you start:

1. MT2L03 with Lollipop installed, TWRP installed, and have a backup of your primary OS (at least /system, and /boot partitions)

2. An empty SDCard that is 32GB or bigger, strongly recommend class 10 or faster. (16GB will work, then you don't have much external space left)

Steps to install 2nd OS to sdcard:

(Note, I will call the empty SDCard as sdcard#2, and your current sdcard in the phone as sdcard#1.)

1. Make a backup of your current OS using twrp, include at least /system and /boot, to your sdcard#1.

2. Install the PartitionWizard (files location below) on windows, and put the empty SDCard#2 to computer sdcard reader.

3. Launch Partition Wizard, and carefully re-partition your SDCard#2 to following:

(pls refer to xda wiki about the usage: http://xdaforums.com/wiki/SD_card_partitioning )

1st Partition: FAT32 primary size: (Total size minus 15000MB), for example, for 64GB(60000MB) card, choose 45000MB

2nd: ext4 primary size: 1800MB (/system for 2nd OS)

3rd: ext4 primary size: 12500MB (/data for 2nd OS)

4rd: ext4 primary size: rest space, should be great than 200MB (/cache for 2nd OS)

Since partition 2/3/4 total size is 14320MB, thus the 1st partition size is (Total size minus 15000MB)

4. Apply the change, this will take some time (like 20 to 30min).

5. Once sdcard#2 is re-partitioned and formatted, unplug and re-plug sdcard#2 to windows again. Right click the sdcard drive letter, and click "Open" to open the FAT32 partition. ( I found double click the drive letter will give error if the FAT32 is bigger than 32GB, but right click and open works)

Copy two flashable zip (file location below) to the sdcard#2.

6. Reject the sdcard#2, and remove it from windows machine.

7. In Mate2 phone, go to settings, storage, and un-mount your current SDCard#1, after it is unmounted, take out the sdcard#1 and put it to a save place. Do not mess the two sdcards!!

8. Put in the sdcard#2 to the phone, and reboot to TWRP.

9. Install the two zip you download in step 5 from sdcard#2. Reboot.

How to go back to your primary OS:

a. Put in sdcard#1 to the phone, boot into TWRP, restore your /boot partition from the backup you took in above step 1. then reboot.

How to switch to the 2nd OS on the sdcard again:

a. Put in sdcard#2 to the phone, boot into TWRP, flash the "MT2L03_xxx_BootOnly.zip" you downloaded in step 5.

Files to download:

PartitionWizard in above step 2:

http://tinyurl.com/q62m68x

location: under Lollipop/SD_Partition_Tools

FileName: pwfree91.exe

This tools is mentioned in XDA wiki: http://xdaforums.com/wiki/SD_card_partitioning

You can use any tool you know to do the re-partition, above tools is what I used and it works.

Customized flashable in zip in above step 5:

http://tinyurl.com/q62m68x

location: under Lollipop/OS_on_SDCards

FileName:

If you want install B309 as 2nd OS, please download "MT2L03_B309_DBoot_BootOnly.zip" and "MT2L03_B309_DBoot_SystemWithRoot.zip"

If you want install CM12 as 2nd OS, please download "MT2L03_CM12_0820_DBoot_BootOnly.zip" and "MT2L03_CM12_0820_DBoot_SystemWithRoot.zip". (Please note the GApp is already included in the zip)

Attached two screenshot of the partition tool, one for 32GB, one for 64GB. Please note though the pic showing in GB, but when config the size, please use MB to make sure the partition size is accurate and big enough.