I'm developing hotels websites here in Brazil. When the site is done, we do an AB test with the old version to measure conversion and show to the hotel owner how good our site is.

Due to the fact that I cannot put the old site inside the new one as a subresource (newone.com/old), currently I'm doing those AB test as follows:

1) I create 2 Google Analytics accounts, one for each site (old and new);

2) I put the GA tags in the old website pages (changing its possibly existent GA ID to the just created one);

3) I put an Javascript code that redirects the user to the old website (in a different URL and different domain) with 50% of probability.

So I compare all the metrics, events and goals between those two GA accounts.

How bad is it? How Google can interpretate the fact of being, sometimes redirected, sometimes don't? The experiment usually runs for 2 weeks.

Is there any other alternative for doing this in a better way?