Since facebook doens't allow redirects, is it possible to split test landing pages with different domains on facebook?
The way i've been doing it is simply duplicating the adset and using 1 landing page for each adset. But that doesn't seem like the best solution since duplicating an adset doesn't always yield the same results as the original adset.
I also prefer not to cloak since this account is 100% white hat and don't want to get banned for something silly. (i know i know, they will eventually ban you even if you are 100% white hat, but still...)
"Since facebook doens't allow redirects" <-- This is plainly WRONG.
To the rest of your question.
Are you using a tracker? If so, do it though the Tracker campaign rotations. See relevant tracker tutorials on that.
If you are not using a tracker, which is far from recommended, then make a folder on your host and an index.php file in it with the following code.
<?php
switch(rand(1,2)) {
case 1:
include("lp1.html"); break;
case 2:
include("lp2.html"); break;
}
?>
Now send traffic to yourdomain.com/yourfolder/index.php
lp1 and lp2 would be your two landing pages. Over a bigger statistical sample, ie more traffic, visitors will be split roughly 50-50 between your two pages.