Hello Medina,
it does not necessarily make a difference to WHICH partition you install the MIUIAndroid Rom. What Hadron was trying to say is, that there MIGHT be errors with the unaltered partition once you've installed MIUIAndroid to one because BOTH partitions use the same Userdata / Apps.
Furthermore, the Original Rom is odexed whereas the MIUIAndroid Rom is deodexed, this might be causing the problems.
Anyways, long story short: You are only using one partition anyways, the other one is left untouched / as a recovery method.
Step 1:
Download necessary / wanted software
- MIUIAndroid 2.12.28 for Xiaomi MI2:
http://files.miuiandroid.com/2.12.28/miuiandroid_aries_jb-2.12.28.zip
- GApps for 2.12.28 JB:
http://goo.im/gapps/gapps-jb-20121011-signed.zip
- Language Pack for 2.12.28 MI2:
http://tinyw.in/uF7e
Step 2:
Preparing your MI2
Make sure your MI2 is charged, turned on, switched to English language and connect it to your PC using the supplied USB cable.
It should come up as a portable Media device.
Open it using explorer - You should now see all the folders (DCIM, MIUI etc), this is called the "Root".
Copy all ZIP-Files (MIUIAndroid Rom, GApps, Language Pack) from your PC to the MI2 in the "Root".
Step 3:
Flashing the ROM
Disconnect the MI2, look for the "Updater" App on your phone. Click it, it will scan for new software and probably say that you've got
a) The most recent version or
b) That a newer version is available
If option b) occurs, select "cancel", because you don't want to upgrade the original software for now.
In the updater screen, it shows the big MIUI logo on top and tells you the MIUI Version below.
Now press the bottom left key on your MI2, this opens a Menu. Choose "Select Installation Pack".
The next screen shows you the contents of your "Harddrive", so to say. scroll down to the bottom until you find the file
miuiandroid_aries_jb.2.12.28.zip
and click this. After that, you have to click the green "Update now" button bottom right.
The system will ask if you'd like to do a backup, but since your phone is left untouched so far, you can skip the backup.
Your update will now be applied, as shown on the status bar. After the update is finished, your phone has to be rebooted.
Once the reboot is complete (which might take a while) you repeat step 3 TWO more times, each time using the next ZIP-File on your Device (GApps first, Language pack last).
So in total, your system should have been rebooted THREE times.
After this process is through, you should have a complete and working MIUIAndroid Rom on your MI2.
~ Klotzi
But flashing it with recovery did not work. I've extracted the zip using root explorer, then I've put the files in there into the corresponding folders and set permissions like hadron showed for 4.2 camera. 
