Common button on two Arduinos and two computers We have two computers giving two presentations that need to    be triggered in sync. I know how to send the space button via Arduino, but I want to check it will be safe for the computers. If each Arduino is connected to their computer via USB, both data pins togeather and both grounds togeather (not sure if I can find a dt momentarily push button in two days). Is there anything I should keep in mind incase both sides are not at the same voltage as eachother?
The ground of both computers are connected also via their power lines. You get a ground loop that might result in malfunctions and/or defects.

Think about using a galvanically separated connection, for example via an optocouple.