Older child has longer name for b/c which we use daily along with various shortenings.
Sasha would literally only be Sasha wouldn’t ever want to use a different shorter form. Love the name Sasha/ Sacha so much and would be disappointed that it wouldn’t be “official” anywhere. But
a) it’s traditionally never a standalone name and I’m a traditionalist
b) life is long; trends change and if he wants to ever use the full name or a different diminutive he could.
only other thing to consider is that we always introduce older child in full name and use it. But would only want to introduce him as Sasha and use that so that might feel a bit unbalanced. And I much prefer Sasha to Alexander
tia