Go inside your product settings >> page settings >> sales page settings >> use split test >> yes
You'll see something like this:
In the example above, there are 2 sales pages I want to split test. I enter their Destination URLs and give them equal Weight (50%-50%).
This means the sales page will rotate evenly. Visitor A goes to sales page 1. Visitor B goes to sales page 2. Visitor C to sales page 1 again, etc.
After you're done with the settings, scroll down to the bottom of the page and click "save changes".
Then go to integration settings tab >> sales page URL. This is your split test URL.