FeatherWing Doubler is a prototyping add-on developed for Adafruit’s Feather platform. The board is very similar to Feather Proto, but you can use two Feathers with this board. The best feature of FeatherWing Doubler is that it allows working with both boards connected to it simultaneously. FeatherWing Doubler package includes one set of Feather stacking headers and 1 set of Feather female headers besides the Doubler PCB.
FeatherWing Doubler has double breakout pins for each Feather pin like the Feather Proto and it has a fair amount of prototyping space consisting of proto holes. GND and 3.3V pins of both pin sets are connected on the Doubler with a full strip of connected pads.
Adafruit Product Page: https://www.adafruit.com/products/2890