The smallest sized insulated box that will allow both of them floor space. You don't want the box so big that they're sliding around inside, but you don't want it so small that they're laying on top of each other.
Use a lot of void fill, (balled up newspaper, enough so that they can't slide around and it should feel just a little tight) and probably in this weather a cold pack taped to the inside of the lid
Also use secure clean bags, securely tied shut (or just use an old pair of socks... (I'm just kidding...))