If you purchase one package, you have to physically "logoff" one device to logon to the other device. You can have as many devices you want, but only one device can be on line at a time.
If you purchase one package with two devices, you use the same username/pin, but only two devices can be on line at a time. In the case of you and your wife with the 2-device package and both on your laptops at the same time, if you want to switch to your iPhone, you have to "logoff" your laptop, then logon to your iPhone. You can't just shut down the device. You have to physically "logoff" for this to work.