It's a cross-share holding - neither are subsidiaries of each other. It was designed I recall as a strategic investment to make sure CX/KA had a strong place in Mainland aviation and CA had access to the HKG gateway, plus management support from CX.

Years ago CX was a partner with UA MileagePlus. I think it is hard to crystal ball if they each stay in separate alliances or if there is a switch - CA to OW or CX to *A. Perhaps if CX left OW, TG or SQ would move in. Pre-ST there was another alliance Wings Alliance which sort of fell apart.