I think you may have alluded to your answer.
It's possible for these 2 tangs to get along despite similar(ish) body types. A lot have people have done it. However since your yellow tang is already the 'alpha' in the established display upstairs; it may not take too kindly to the Kole Tang.
Conversely; you may have a better chance if you introduced the yellow tang downstairs as it is a new environment for it. I know...that's not the point.
Prepare for some bullying and monitor to see if it gets out of hand. The kole tang would generally be more docile when compared to the yellow tang.