Can one MacBook charge another over USB-C?
Yes you can. I finally figured out how. 

Use a dongle and hook them up like this:

power supply &gt; usb-c cable &gt; mbp &gt; usb-c cable &gt; usb-c multi-port dongle &gt; mbp2


I've tested it with the UpTab displayport with power delivery adapter and with the Apple multiport adapter (A1621).

In my experience, 15w is enough to power and charge the 15" mbp.
Here are discharge rates I've observed on my laptop:


Idle w/min screen brightness: 4.25w
Idle w/max screen brightness: 7.7w
Chrome open, no tabs, max brightness: 8.6w
Chrome open, 2 gmail tabs, 1 pandora tab, max volume: 16.5w
Chrome open, playing 1 YouTube video, min brightness: 9.4w
Chrome open, playing Spotify, min brightness: 6.05w
iTunes open (Chrome closed), playing song, min brightness: 6.1w


The second mpb will only receive 15w from the first. An app like coconut battery can help you see how quickly your laptop is charging.

Using this method, you can even trick your laptop into thinking that it is charging itself. (And then you could upload that video on YouTube, claiming to discover infinite energy, and profit on the ad revenue)

The OEM power supply for the 2016 15" mbp has 3 output modes:


4.3A @ 20.2v = 86w
3A @ 9v = 27w
2.4A @ 5.2V = 12.48w


So even the OEM power supply will occasionally supply only 12.48w to the mpb.

Update:
After playing around with this more, the above method will work if you want to charge 2 identical devices. But if you have a 15"mbp and a 13"mbp, it seems like the larger device always supplies power to the smaller device. I don't have one available currently to verify exact charging watts. Charging a 15"mpb from 0% to 100% battery takes about 8 hours at 15w. This assumes that the laptop is off or is sleeping. In a pinch, I've worked 8 hours with my 15"mbp plugged into my 18w usb-c phone charger. I was able to maintain the battery's charge during light to medium load. The move to usb-c for power has been awesome. You could conceivably charge your laptop with the 5w iPhone charger, but it would take about 15 hours to fully charge while shut down (assuming a 75wH battery and 5w charge rate and no inefficiencies due to charging), I'll have to test this out tonight to verify that it works.