If you stream both at the same time, make sure the combined streaming speed of both of you does not exceed the upload of your internet connection. You also have to count any upload bandwidth the game you're streaming uses and any voip traffic both of you may use at this time.
Your router may show a "used bandwidth" value or graph, but ususally this is only a very rough number. If the combined upload speed of both of you exceeds the upload speed of your internet connection, you're getting lost frames at the same moment.
If both of you are streaming via wlan, it may very well be that the wlan is the bottleneck and not the internet connection. In this case, use LAN cable.