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.
As long as you don't have to send anything from the pcs to the arduinos (or at least not with both) you can simply connect the arduinos TX with both pc's RX. This way you need only one arduino. Use series resistors (1k) to handle voltage differences between the pcs. If you need two arduinos you're fine without galvanic isolation, but I recommend series resistors on the reading gpios as well (just in case you configure the ports as outputs accidentially). In case you use laptops which usually don't have PE, you have to connect the arduinos grounds as well.