sundarrawat - 6:22 am on Oct 10, 2013 (gmt 0)
Redirecting via 301 or 302 is not a good idea as sometimes 302 redirection does not redirect completely and shows error. And, 301 is not good as you have to test only...
As your purpose is to optimize Conversion Rate then I would advice a different way.
I would advise you to create a folder in Site A root - "new"
Now get a copy of 'site B' and upload on new folder.
Now use 'Google experiment feature (A/B Test)' to test Conversion Rate. I also would suggest you to use 'VWO' where you will find 'Split Testing Feature' which is more insightful.
Via this, you ranking will not be affected.
And after result, if you find that site B in 'new' folder is performing better than you can stop the testing and redirect site A to site B completely with 301 rule.