BaronAfanas Google would know those two devices are owned by the same person, and thus that any data collected from each of them are about the same person. That's about it. This could be a big concern if you intend to use the two devices for totally different kinds of activity, that you don't want linked, such as banking on one device and activism on the other. But it shouldn't be much of a privacy concern at all if you pretty much use the devices for the same kinds of activities, such as if it is your phone and your tablet, or your old phone and your new phone.