Hi Ben,
You can create an XML file with the two data fields. For example:
<?xml version="1.0" encoding="UTF-8"?>
<Util>
<Fields>
<FName></FName>
<LName></LName>
</Fields>
</Util>
Add the XML file as a receive type data connection (select XML in the wizard). Then, do the following:
1. Open the check box properties and add a new rule.
2. Set the condition such that check box = TRUE.
3. Add Action to Set a field's value.
4. Select to set field1 from the XML source file so that it takes the value of field1 in your Main data source.
5. Repeat step 3 to set field 2.
6. Edit the email data connection subject value, replacing the fields using the one in your data source.
Let me know if that works!
Thanks,