How do I properly use the canonical tag to avoid negative effect from having identical content on 2 url’s?

[swiftseo]

To illustrate… I have same website uploaded at 2 locations (url’s). Only the domain extensions are different.

www.myexample.com
 www.myexample.org

The benefit is that I may run some promos on one location and not the other to help in product surveys/testing. The website content is 98% identical and I understand this content duplication may cause SEO problems. The domain I wish to use for rankings etc is www.myexample.com

1)    How do I go about avoiding seo problem?  Do I need to place the canonical tag at www.myexample.org ie

2)    Do I also place the exact same tag at the .com location or not necessary there?

3. Is there an alternative or more effective option to resolving the problem?

[RobertFisher]

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,