Differences – ‘Optimal’ vs ‘Ideal’

As a native English speaker, I feel they have different implicit meanings. Without knowing the context, this is how I interpret them:

• I'm going straight home after work. How about you?

  I'm going straight home after work, which I assume you are also doing, would you like to accompany me as far as it is possible to travel together? Or, can I give you a lift?

• I'm going straight home after work. What about you?

  I am going straight home after work. You are not invited to join me, and I am not interested in doing anything with you, but I expect you will not be going straight home and I am curious where you are going instead.

John Lawler put it excellently in his comment above: "'What about you?' requests a statement about you in general, while 'How about you?' requests a response about your manner, means, or condition."

There are two different idioms here, and you may be confused. In Don't all speak at once, all and at once are separate, and both modify speak; the normal rules apply to all/both, as Barrie has said. But all at once can also be a single phrase, where all intensifies at once ("When all at once my heart took flight", My Fair Lady). I don't believe there are any rules for when you can or can't use that.