Metal and wireless signals tend to not mix well, and having two metal barriers will really knock down a signal. Signal boosting may help, but a better approach is to concentrate on the receiving end of things.

Try a wireless bridge such as Linksys' WET11 or WET54G, which use an Ethernet connection and can be positioned up to 100 meters away from a connecting device. Try to at minimum position these devices in a window that has line-of-sight to the device that you're trying to connect to. You might also have to connect a higher-gain antenna and mount the antenna outside the building to get a successful connection. If you use a higher-gain antenna, keep the cable length as short as possible, since thin consumer-grade antenna cable can attenuate signals much more than you would expect.