You will need one card per person for travel

Technically, you could probably "double-tap" on a bus but you cannot do that with the MTR. The system calculates your fares based on distance (i.e. stations you tap in/out) so you must tap out before you can tap in again.

You may be able to tap out (without exiting) immediately after entering the paid area and then pass the card to your wife to enter, but even if it worked it'd be fare evasion and discourteous to other passengers and probably not what you were looking to hear anyway. Trust me, once you get there you will see why Octopus is so ubiquitous in HK.

Probably not. There's probably a mechanism built into the bus/tram/ferry Octopus terminals to prevent reuse of the same card within a certain period of time, if for no other reason than to prevent inadvertent or accidental double-charges in the normal use situation.

This fee is waived for the Airport Express Tourist Octopus, since it's designed up-front for use only by short-term tourists.

The time is 2-5 minutes. I've had to do that a few times. Just get a separate Octopus Card for each person.

Just curious about why someone would think this.

Actually I have used one system where this is possible (Seville) and I was really confused at first when the attendant told me to tap on the entry gate several times while each person went through... anywhere else in the world where multiple people can use the same card?

Also, it's not advisable to double tap on a bus in HK if you're unfamiliar with it, because it might be a route with cheaper fares for short journeys. (Though I once got a driver to refund us in coins!)

On certain bus route with zone fare (6, 15 on HK side that I am familiar with) You tap to get on and when you get off you tap again to get the fare difference between a full trip and your portion refunded.

Yes this is what I am talking about. If you double tap (waiting 2 mins in between) to pay for 2 people then it will screw up the refunding.

Thinking about it, I should have probably paid extra as I paid for one short journey instead of 2 long journeys, but somehow the driver decided to give me more money :D

There are a handful of taxis that do.

For red minibuses, those that run set routes do have them. For example, between Mong Kok and Kennedy Town - all of them take Octopus.

Well, in Paris one person pays and four people use the ticket! :D

I meant complete implementation.

This can work in practice only if there is a flat fare regardless of distance traveled. The NYC fare cards probably allow for this, if you were to swipe once, go through the gate, and hand the card back through the gate to the next passenger.

No, it just requires the card to have an extra field for "number of passengers" whcih is incremented by touching an entry reader and decremented when touching an exit reader (with deduction of maximum fares or whatever). I assume that the card and all passengers make the same journey, otherwise it gets too complicated.

Does anyone know a way to liquidate or cash-out an octopus card? Can you transfer to a HK checking account for example? thanks

You can just go to a MTR station and cash it out at the counter. They'll give you your money back, along with the deposit, minus I think $8 or $9 HKD if you are returning it within 3 months of purchase.