Use the following scripts to install apk on multiple devices/emulators.
for SERIAL in $(adb devices | grep -v List | cut -f 1);
do adb -s $SERIAL install -r /path/to/product.apk;
done
Remove -r if you are not reinstalling the apk. Also you can replace "install -r /path/to/product.apk" to other adb commands like working on one single device.
It works for me on real devices but I believe it should also works for emulators.