I wouldn't class a iframe solution the best solution, while this doesn't answer your question directly it does give you an additional and more robust never-mind a more browser friendly site.
The method that I believe is best suited for you is to use "Rel Canonical" on the .gr domain referring to .org as the master (No Redirects). If they visit .gr they remain on gr and vice versa on the org. The org will be the master for rankings purposes in Google as same as the iframe method.
So to make this possible you do the following:
- Clone the file contents of the site to another hosting folder so effective you have 2 hosting accounts but these are mirrored.
- Setup the .gr site to use the database of the .org site so both sites are exactly the same. (You may need to adjust the base_url) or use a multiple site plugin. (if you don't have a SQL then even easier).
- You will want to use Rel Canonical on both the master .org and .gr as the slave.
- Optional step (You could even robots the .gr site from indexing)
Examples of Rel Canonical in Action
In the head of your master site (.org domain) you would use the following:
<link rel="canonical" href="http://www.example.org/page1"/>
In the head of your slave site (.gr domain) you would use the following:
<link rel="canonical" href="http://www.example.org/page1"/>
This basically tells Google that the master is the org, you could even go one step further and robots.txt the .gr site completely but with this method you will not get a duplicate content slap on the .gr with or without robots.
Using two domains one site WordPress
Step 1: Go to your General Settings and completely remove the "Blog address (URL)" and save.
Step 2: Add something like the below code to header.php
$ThisDomain = $_SERVER['SERVER_NAME'];
if($ThisDomain=='example.org' || $ThisDomain=='www.example.org')
{ echo '.org stuff'; }
elseif($ThisDomain=='example.gr' || $ThisDomain=='www.example.gr')
{ echo '.gr stuff'; }
Step 3: Ensure HTTP_HOST is not set in the SQL