Because the guy who plays Joe.1 is a fantastic actor, too good to be wasted on just mirrors.
Sure, so he could do the role of joe 2 and the other guy the joe 1, since joe 2 get more screen time. I just don't think the way they have done it was the best. But i think i'm the minority in my opnion.
What was the point in the revenge if he would go back in time? In the end, Park Tae Woo was free again... not to mention the the serial killer painter...
https://kisskh.at/729705-hidden-love
(non spoiler required, tvm)