BoA and Sung Si Kyung sang each other's hit songs.

On the July 25th installment of 'Knowing Brothers', the two legendary solo singers will appear as special guests. In the pre-released clip, BoA and Sung Si Kyung are seen live-singing the songs that were quintessential in their respective careers. 

BoA sang "Two People" by Sung Si Kyung, and he, in turn, sang a short medley of BoA's "No. 1" and "Only One". While BoA was singing the ballad, Sung Si Kyung joined in and the two ended up delivering a brief duet!

Stay tuned for the full episode today at 9 PM on JTBC!