David; you can trigger a rule from a button like this:
<button type="button" onclick="spjs.dffs.triggerRule(['Append']);">Test</button>
My ‘Append’ rule sets the value of Box_1 to “{Box_1} {Box_2}”.
So, when Box_1 = “One fish” and Box_2 = “Two fish”, after pressing the button on my form, Box_2 = “One fish Two fish”.
If you want to insert a line break when appending, your rule would look like this: {Box_1}\n{Box_2}
Based on my testing, that will work for either plain text OR enhanced rich text fields.
NOTE 1: DFFS is incompatible with Rich text fields, as far as I know.
NOTE 2: I tested using the Dec 14, 2015 version of DFFS.