Official comment
One: Create a group with the criteria field [Contact ID] ends with 0 or 1 or 2 or 3 or 4. It should split it almost perfectly in half. If you plan on sending all of the versions of a message date sequences work well.
This method also works well to segment your contact list into tenths; you could send one test to the [Contact ID] numbers ending in 0, and the other test to the [Contact ID] numbers ending in 5. When the split test is over and you know which email works best, send it to all Contacts with the [Contact ID] ending in 1, 2, 3, 4, 6, 7, 8 and 9.
For more information, check out our Knowledge Base.
If you're an ONTRAPORT User, you can access an ONTRAPORT Project step-by-step training on setting up split testing in ONTRAmail. To do so, navigate to this link and the project "Split Test Email Subject Lines to Improve Open Rates."