The Split Test Email action allows you to set up a split test email send scenario for those contacts who enter the Journey's funnel. See Fig 1 below. A given contact who enters the funnel will receive one of the designated emails you add to the Split Test Email ruleset. 

Fig 1

To create the split test ruleset, click the Split Test Email action node once you have added it to the Journey work area. Doing so will bring up this popup ("Properties") on the right-hand side of the screen (See Fig 2 below):  

Fig 2

Clicking the "Campaign" input field below the "Split Test Email" label will bring up a list of all your available emails from which you may choose the first email for the split test (See Fig 3 below). Select the email by clicking its name.

Fig 3

Once the email has been selected, the system will autofill the inputs for (See Fig 4 below):

• Subject
• Send-From Email
• Send-From Name
• Reply-to Email

Fig 4

Please note that you may edit the values for :

• Send-From Email
• Send-From Name
• Reply-To Email

Once you have finished with the email setup, click the "Save Split" button. The system will now allow you to repeat the process with a second email (See Fig 5 below):

Fig 5

You may add as many emails to the split test as your needs require. Once you have added all the emails to the test, click the "Save Properties" button.

You will note that to the far left of each email name, there is a toggle switch that is "On" by default. Clicking it to "Off" will remove that email from the split test send.