The only way I can think of doing this is to calculate your previous years trendlines, then export them (worksheet > export > crosstab to excel), and then re-ingest that trendline into your viz, edit your relationships and build a dual-axis viz.
In this example just to make the trend lines different I've added a random number to the second mark but you can see how this will create the 2 trend lines you are looking for.
HTH 2018.1 attached
Divya.twbx 8.9 MB
In your solution, it is showing all years I want 2013 (prior year) year sales& 2014(latest year) year sales monthwise.
I am not able to open your file as it is in the newer version.