Comparing 2 best fit lines for 2 halves of a data set

#1
I have a data set with ~200 coordinates plotted on a scatter plot. I can either use the data set as is, or split this plot into two data sets along a line x = 0.2 (so that the first new set will have the range -1 to 0.2, and the second will have the range 0.2 to 1). I then create a best fit line for each data set (one for the whole set, and one for each half).

How do I compare between which best fit line works better? I am comparing between using 1 best fit line for the whole data set, versus using 2 best fit lines, one for each half of the data set.