Hello, is it possible to power a arduino by connecting another arduino's 5V and GND to the Vin and GND?
No.
However, if you connect the +5v of one to the +5v of the other, as well as the grounds, then yes, it will work.
Vin requires more than about 6.5V for the Arduino to work right. But bypass the regulator completely and go straight for the +5v pin, and you should be fine.