We've seen that Urek doesn't try to amplify his overall shinsoo power through exponential means like baam and the fh's do by collecting artifacts, weapons, etc(for example urek didn't take the red thryssa). Urek also didn't learn shinwonryu. And we know Urek needs to beat phanta, so as a result he needs to obtain as much power and techniques as he can for that mission. If he can merge his shinsoo ability with this unique energy of his, wouldn't he also want to make his shinsoo as powerful as possible? Now if shinsoo doesn't add any additional power on top of his unique energy, then it makes sense as to why Urek never learned shinwonryu, collected artifacts, weaponry, etc. It's useless, it's not going to make him stronger overall because he can't use both of his energies at the same time, so he never bothered(this mysterious energy he uses probably amplifies his power far more than shinsoo can) . Plus outside the tower you can't use shinsoo and that's where Urek would probably fight Phanta if it happens. So it wouldn't make sense if Urek could merge both imo.