I have 2 theories on this Sub-Zero situation, and 1 of them will please the fans of both brothers, while the other may cause a riot.
Theory 1:
Sub-Zero in MK X is BOTH Bi-Han and Kuai-Liang via variations ("Grandmaster" = Kuai and "Cyromancer" = Bi-Han), just the same way that MK9 Sub-Zero is both Bi-Han and Kuai-Liang, via alternate costumes.
Similar to Cassie Cage, where 1 variation is Sonya and the other variation is Johnny.
If this is the case, maybe Bi-Han survives the Soulnado and Kuai's soul somehow comes back too.
Everyone wins.
Theory 2 (which may start WWIII, here, on TRMK):
Sub-Zero in MK X is a relative of the other 2 brothers or a former student, and picked up both types of fighting styles ("Grandmaster" and "Cyromancer") from Bi-Han and Kuai-Liang whilke training in the Lin Kuei, ala Cassie Cage.
Thus, making THIS Sub-Zero a NEW Sub-Zero, and neither brother.