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Swiftseo I, personally, would not have two sites with varying domains that are near identical. No matter what the reason, I think there is more negative than positive. I can name multiple reasons for why using this for testing is flawed, first and foremost.  I would pass on the canonical and instead use a 301 redirect of "testing site" to main site. That way, if there is anything in the way of link juice to pass it gets done. If you need help with how to on 301, there are various places to go or place a question in Q&A. The main thing you are hurting with this methodology is yourself. You have two sites competing against each other. hope this helps,