I have two laptops; each has a USB-C port that accepts a 65W power supply, both running Windows 10.
Say I have laptop A fully-charged, and laptop B is nearly dead. I'd like to plug laptop A into B with a USB-C -> USB-C cable, and have A charge into B.
Is this possible? I feel like I've seen screenshots of this, where it prompted if the device should act as a power source or sink; but I can't get anything to be recognized.
No.
USB-C does not dictate what power levels a device can output except for the default 5V out, while most laptops require 15V or 20V in order to charge. (Or would require double or quadruple the current to charge, with larger and possibly unreasonable drops in conversion efficiency due to needing to first step up the voltage to be able to push electrons into the battery)
USB PD (Power Delivery) is a negotiated power delivery protocol, devices advertise what voltages and currents they support, and can request the most efficient method they desire. If the device does not support outputting the required voltage then they will fall back to the default 5V which may not be enough.
It is this negotiation that makes most USB C chargers "safe" to use across devices like phones and laptops. They will output 5V by default, then if neither device supports a particular power profile (voltage/current) then another can be selected. Either the devices find a high-power profile they can work with and select or they get the default 5V.
You would have to look at the specifications of both laptops to find out if they support USB PD output, and check that the output of one matches the required USB PD input of the other. Just having USB C does not guarantee that it supports USB Power Delivery.
As an example you can easily find battery pack power banks that have USB C, but only support up to 9V output which is not enough to charge a computer but is fine for compatible phones. Then there are high power 60W devices that support (and list) up to 20V at 3Amps which allows them to charge many newer laptops.
Many laptops already have power conversion circuitry to go from 15-20V power supplies down to their battery voltage (usually 7.2V) for charging. That circuitry is not generally bi-directional and doesn't necessarily mean that it can convert the 7.2V battery voltage back up to 20V for output. They would require extra electronics to output that voltage and power level.
If your laptops do not specifically mention USB PD Output then I would assume that they can only put out 5V on the USB port.
