I was thinking about this and I think it's pretty likely that Si Woon is the seme in this particular development of their relationship.
There was a special chapter released on Lenzhin showing what the two would have been like had they never separated and Tae Min never lost his body, and I think if that were their current reality then Tae Min would have likely been the seme, but not now. At least, that's how I've been interpreting it. I won't be upset if this manhwa proves me wrong though! I'm down for either really.
Basically, I summary of how I see their relationship via:
Reality where they never separated and Tae Min never dies: Si Woon is always tailing by Tae Min's side, has grown confident, but is at the service of Tae Min. (Tae Min=seme, Si Woon=uke)
Current reality: Si Woon is arrogantly confident, coaxes Tae Min to remain by his side, and desires to service Tae Min. (Tae Min=uke, Si Woon=seme)
I actually don't read manhwas and stick to manga but the picture of Victor from YOI caught my attention. After reading this, all I can say is that 'o'. This is just gold. The plot seriously is getting good. I'm a little disappointed though when I saw it's just shounen ai *sighs* no bum sex. Nevermind, things are getting instense and interesting that I'm dying to read the following chapters. I'm wondering about something though. If Tae Min is still in his body, I wonder who would be the seme?