What would happen if I attach a Raspberry Pi (A) to a USB hub for power which is already connected to a second Raspberry Pi (B) powered by an adapter (but connects to devices such as HDD, keyboard, etc., via USB hub)?
Could this damage B? What will B see as A when I type lsusb?
If said hub is powerful enough to power both, nothing bad will happen. As stated in other answers, not all hubs can or will provide that much power, what may lead to the Pi malfunctioning/power cycling (but not burning).
It depends on what type of USB port you have. The USB 2.0 specification allows Hosts to deliver 5V at 500mA, for a total power output of 2.5 watts. USB 3.0 and 3.1 allow 5V at 900mA (4.5W). Certified Hosts and Devices must limit their power delivery and consumption to these "default" power levels. This is from the Tripp Lite website, link following. There is a lot of good information on this website "https://tripplite.eaton.com/products/usb-charging"
Powering a Pi from a USB port is not recommended as the USB specification only allows for 0.5 amp well below the suggested supply current of 1 amp for a Pi model A. There are however cases of this working, but there are an equal number of hard to track down bugs and faults caused by underpowering the Pi.
Since the power port on a Pi only connects the power pins and not the data pins nothing bad will happen by connecting it to a hub that is already connected to a separate Pi. You can verify this with the schematics.
Maximum total USB peripheral current for Pi(2/B+) is 600/1200mA. The limit for Pi3 is 1200mA. Earlier models claim 500mA.
The default for 2/B+ is 600mA which can be doubled by setting max_usb_current=1 in /boot/config.txt.
See Raspberry Pi Power Limitations
It should be possible to power another Pi, provided the total current does not exceed 1200mA. It is unlikely that you could power a HDD as well.
My experience is that even running a HDD from the Pi3 causes reliability problems, so you should use a powered hub with the HDD.
The actual performance depends on the hub. Some impose limits, although most do not, however many do not supply 5±0.25V.
In summary, you COULD run a Pi from one of a Pi2/3 on-board USB ports, given a good PSU, but probably still need a powered hub.
