A = X
B = X
A = B
John = human
Steve = human
=> John = Steve
obviously wrong, but how exactly?
But in either case the argument would be valid (not "true", "valid"). If it were true that "John" and "human" were names for the same thing, and that "Steve" and "human" were names for the same thing, then "John" and "Steve" would both be names for that same thing.
Now, is your question whether that argument is "true" (the conclusion is true argument) or "valid" (the conclusion is true if the premises are true).