This works now:
SELECT @a \\:= 1, @b \\:= 2, @a + @b
Thanks to Strong Liu's fix:
However, you need to modify the query, which I don't like as it's not "native" then.
It should be fixed by looking ahead for the '='.
But that would be much more complex - := is MySQL specific, and therefore implementing that would involve moving this exception to Dialect.