What happens when Warwick ults Pantheon or a champion with a global ult while channeling his ult, is it possible for both of them to go up?
Depending on the timing, it is, however possible for skills that aren't cc, this can happen:
Given that WW ult is also a suppression, though, you're most likely just stop the jump. With some skills that move a champion (Ezreal's blink, Shunpo, etc), WW will start ulting and flailing around where they were, but they're still cc'd in their new location.
Not in this case. If CC like suppression (Warwick's ult), stun, silence, knockup, fear, polymorph, knockback, or in short everything except slow really is applied on someone who is channeling, their channel is interrupted and it is cancelled.
Various things with channel are: Fiddlesticks Ult, TF ult, Pantheon ult, Teleport summoner skill, Fiddlesticks drain, Nunu ult are all affected similarly. Shen ult if interrupted will stop his teleport to the ally but the shield he gives is immediate.
That said if an ally ults, that can work. For instance Shen can ult onto his ally Pantheon while pantheon channels his ult.