You make your string in a microflow. I imaging your first iteration does:
string x = "first line"
What you have to do for the second line is
x = x + " second line"
Note(!) the line-break in my second statement. HTML / HTML entities e.t.c. is not supported yet, you have to make a real line break in your variable change action.
N.b.: There is already a feature request in our Partner Portal for a HTML widget in document templates.
see this question or this question
'%0A' don't work either!
Nothing works! newline, etc!
I had the same problem when building an email message for an external link with HTML mailto:...
Use 'First line' + '%0A' + 'Second line'. This should work.