I’d just like to leave a little note: In case you’d like to see all player windows at the same time, you might want to add
-ResX=1280 -ResY=720 -WinX=0 -WinY=0 -windowed
for a first client and then
-ResX=1280 -ResY=720 -WinX=1280 -WinY=0 -windowed
for a second client. This allows you to open the clients in windows, and set their position and size. At least in UE4.24.3 that works for me.