One simple way would be to export your entire list, and perform a Modify Existing Records import, via you Admin>Data Cleanup options.
You would pull up your entire database, select everyone, and choose Export, under the actions menu.
When setting your export up, you will ONLY need to export the Contact ID column. You do not need to export their name, email, or any other data. Just the contact ID.
Once you have exported your contact IDs as a CSV file, you can quickly open it in Excel, and add a column called ‘Tags’. Then, copy and paste one tag (ex: SplitTestA) to half of the spreadsheet, and a second tag (ex: SplitTestB).
Save your CSV and head to Admin>Data Cleanup, and choose the option ‘Modify Existing Records’. The process is in line with a normal import. You will choose to modify import ‘Contacts’ upload you CSV of Contact IDs and these tags.
Next you will configure the Field Matching. It should automatically choose the correct options but ensure that the Contact ID field is matching to ‘Infusionsoft Contact ID’ in the drop-down and that Tags is matching to ‘Create Tags based on the value in field’
You will reach the final step where you can begin the modify existing records import. Since you are only presenting the contact ID, and the two tags, the system will just add the tag to the appropriate record, and all other information existing will of course still be there.
Once this is done, half of your list now has one tag, and the other half has the other, allowing you to split test.
Here is a help article that goes over the Modify Existing Records import that you may find useful Sort contacts automatically
There are also some options through 3rd party solutions that can assist with future split testing automation.