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.
Two: You can use the same logic as above but with rules and an add to sequence rule outcome. It gets a bit uneven if you have multiple points of entry and not all of the points hit the filtering rule.
If you only have one point of entry and want to automate it moving forward, you could use one split tested page as the entry point. Have one form that leads to Sequence A on Version A then a different form that leads to Sequence B on Version B. One-half of visitors will see A and one-half will see B.
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."