It could be CPR. Erhapspay Igpay Atinlay. But what is the most useful skill that:
- you have learned?
- you want to learn?
- one could theoretically learn?
Given a loose two hour time constraint?
The knot they use to tie shoes.
how to operate a motor vehicle
Well, if you already have experience in another vehicle that is doable, but if that is for beginners… Well, that can increase the number of organ donors, and I do think learning to be a medical courier can be done in a short time.
To become a medical courier, you need a valid drivers license and clean driving record. Training is not always required, but it is highly recommended. Private medical transport companies employ the majority of medical couriers, but there are sometimes jobs with hospitals, medical facilities, or laboratories too. On average, medical couriers are paid about $12 per hour or $47,000 per year.
$12/hr is not $47,000/year; it’s about $24,000/year.
50 weeks x 40 hours = 2000 hours x $12 = $24,0000
Even with no vacations, it would only be $24,960/year.
To make $47,000 at that hourly rate in a 50-week work-year, a person would have to work 25.55 hours of time-and-half overtime each week, for a whopping 65.55 work hours each week. Assuming 1 day off, that would be 11+ hour workdays every single week.
Well, yea, those Jobmonkey people are monkeying around 
, but it is something that can be learned in a short time.
Well, in 1986 I was taught how to use a Mac, how to do word processing in the free MacWrite that was on the floppy disk they handed to us, including setting margins, cut, copy and paste, printing, setting tab stops, saving document, saving changes subsequently, opening an existing document, etc.
It gave me sufficient experience with the general modality of icons, mice, menus, clicking and double-clicking that I self-taught myself an immense amount of additional Mac-related skills from trial and error and common-sense extension of what I’d been shown.
Learn to drive a clutch. If you ever decide to rent a car in another country, you want to have this in your skill set.
Lockpicking
How to make bread. How to sew on a sewing machine. How to milk a cow. How to butcher, pluck, and clean a chicken.
That’s about how I learned to do spreadsheets. The professor wanted an assignment done on a spreadsheet, so she took about half an hour out of a lecture one day, and taught us pretty much everything we needed to know about Lotus and Excel.
Yeah, I was thinking about that, as I stare at my lock picking tools on my desk. I was wondering if two hours is enough. I mean, I guess you can learn to rake a lock in that time, like something like a cheap Master padlock. Raking might help you with basic locks, and two hours might be doable for that. To get good at it and then get into single pick picking, and dealing with anti-picking spools and all that – that takes a hell of a lot of practice.
Breastfeeding a newborn infant.
Doubt if I could learn that in 2 hours.
Assertiveness skills.
In less than two hours you could be taught how to be supportive of a mother learning it, whether she is your partner, sister, daughter, client, neighbor, customer, parishioner, friend, relative, bridge partner. It takes a village.
Fighting ignorance and all.
I don’t know if it’s “most useful”, and I certainly didn’t reach mastery that quickly, but…
Some here might know that 3D design is a hobby of mine. My introduction to it was someone telling me “Here’s Tinkercad. You’re going to be teaching a bunch of middle schoolers how to use it in about five minutes.”.
You Take That Back!! 
In this house, we OBEY the Laws of Thermodynamics! Which one might learn in two hours, to modest benefit.
^ Dr_Paprika’s first name is Homer. 