Create a custom field called "SplitTest" or whatever you want to call it.
Open your webform inside Infusionsoft.
Click "Field Snippets" - Add that custom field as a hidden field to your form.
Save / republish your campaign.
On your website, add the custom field to both web forms. The "name" of the field must match the custom field name and it is case-sensitive.
<input type="hidden" name="inf_custom_SplitTest" value="1">
<input type="hidden" name="inf_custom_SplitTest" value="2">
So basically - you hard coded a hidden custom field in your form so you know which one was submitted.