To address the actual topic of the thread for a change: simply supposing for a moment that the premise holds, and that Alexander lives long enough for a confrontation between him and Chandragupta to become possible....
Would this happen? I personally rather doubt it. Regardless of what certain overly deterministic minds like to imagine, the very presence of Alexander (and the continued existence of his Empire that follows from that fact) would influence Chandragupta's decision-making process. In OTL, it was highly opportune for him to drive West into the outlying regions of the Seleukid Empire— which was distracted due to the Wars of the Successors being fought mainly to the far West. This allowed Chandragupta to gain what he wanted, and then to make a peace on his terms. Very profitable indeed.
If Alexander is still around, such an invasion would very simply be less tempting. In fact, it would be a big gamble, and Chandragupta was a man of planned and deliberate actions. As I have argued before, the logical step for him would be to demand that Alexander's vassals east of the Indos simply start paying tribute to him, too. No Western expedition beyond that. Instead, he'd do well to subdue the entire Indian subcontinent. That was his plan in OTL, but the opportunity presented due to the Wars of the Successors was just too good to pass up. In this ATL, he very likely sticks to his original plan. when done, he'll realise that no action is forthcoming regarding his leaning on Alexander's Indian vassals, so he may annex those, too. Still no moves beyond the Indos, though.
Why not? Because while Chandragupta has been campaigning in India, Alexander has been campaigning in the West. Again, there are determinists who maintain this could never succeed— but again, they are wrong. The end result is that we have Alexander and Chandragupta, both having conquered all they wanted except anything the other has, and now consolidated in their positions. The option they both have is to start a direct war with the only real rival in the immediate vicinity, in a war that may well prove to be an existential threat to both. Would either be so stupid as to invade the other? I have my doubts.
If either one starts an invasion, that's very likely the one who loses. There is always the risk of the defending party just dying by chance during the war, which then scatters his followers, allowing the invader to win... but the likeliest outcome is that the invader (suffering from immensely stretched logistics, being very far from the other's imperial core) loses to the defender (who has the home advantage). No matter what, it would be a spectacular clash... but it would also be a contender for "most pointless war ever".
As we all learned from that one film: the only way to win is not to play...