A: In order to send one email with two different subject lines, below are the detailed two ways to achieve this:
A/B Campaign: It can be done by creating A/B campaign. Select a test list and then on the second page of A/B campaign creation page, you can select the same content in both the groups and give different subject lines. By doing so the campaign will be sent out to two groups with different subject lines.
Workflow: It can be done through workflow by using A/B Split element. Double click on the "A/B split", name the group and enter the percentage as per the requirement and attach the element with "send Email." Double click on "send email" and enter a subject line. You can create multiple groups for multiple subject lines.