So I checked this theory...
and the result is: YES IT IS POSSIBLE
It is advisable that the TVs that are going to be combined were at least the same brand, that way your pinout should be some what similar.
Unit that I worked on were both made by SONY.
The logic board donor is 32" Sony KDL-32M3000 http://store.sony.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10551&storeId=10151&langId=-1&partNumber=KDL32M3000
Recipient is 40" Sony KLV-S40A10 http://www.welectronics.com/tv/SONY-KLVS40A10.HTML
So the little one had a broken panel but everything else was working fine. I even have a remote for it ;-)
The big one had a bad logic board where 5V stand by turned into 2V and 17.5V into 8V. The power supply and the inverter board were OK.
I couldn't upload videos to this instructable so I am posting an external link to them.
Now let the madness begin...
Step 1: Check If LCD Cable Is Compatible
How I did that is by turning on the little TV and connecting the LCD cable from main board to the panel of the big TV.
The very first hint is the LCD cable should be physically compatible.
After that I put 5VSTB into POWER-ON which turned the backlight ON.
If signal is compatible to the LCD panel then you should see the picture.
What I saw is the SONY logo at the loading state.
That is the clue!
Step 2: Taking Apart Sony 32"
P.S. Originally I thought I could use the PSU from the big TV to power the new main board but it didn't work out.
Step 3: Taking Apart Sony 40"
After that found the wire to start the back light and the jumpstart the PSU it self.
Step 4: Finding How to Jumpstart the 40" PSU Without Logic Board
Step 5: Moving Everything Into 40" Body.