It is going to be interesting to see how this plays out because if Yoon and Ahn merge their campaigns they have a better chance of defeating Lee for the presidency:

The main opposition People Power Party (PPP) faces growing pressure to form a coalition with Ahn Cheol-soo of the minor opposition People’s Party for the March 9 presidential election, as surveys show that an alliance offers a greater chance for the opposition bloc to beat ruling Democratic Party of Korea (DPK) presidential candidate Lee Jae-myung.
Korea Times
So far, the PPP has been downplaying the recent rise in Ahn’s support rate, saying it is a “temporary phenomenon” stemming from infighting in the main opposition party between its presidential candidate Yoon Suk-yeol and party Chairman Lee Jun-seok throughout last month. However, anticipation is growing that the main opposition party may make a decision in the coming weeks in order to grab the public’s attention before the Jan. 31-Feb. 2 Lunar New Year holidays.
According to a Realmeter survey released Monday, the DPK’s Lee was leading the pack with a 40.1 percent support rate, followed by the PPP’s Yoon with 34.1 percent and Ahn with 11.1 percent. It was the first time that Ahn logged a double-digit number in the weekly survey conducted by the poll agency at the request of OhmyNews.
You can read more at the link, but what is even more interesting about the polling is that not only is a combined opposition ticket outpoll Lee Jae-myung, but if Ahn Cheol-soo is the lead the ticket polls even better than if Yoon was the lead candidate.