Might be an easier way, but I just sent a keystroke for CTRL+N for a new window before sending the next start command. This works for me. (save as a .ps1 for powershell)
start microsoft-edge:http://google.com
$wshell = New-Object -ComObject wscript.shell;
$wshell.AppActivate('Google - Microsoft Edge')
Sleep 2
$wshell.SendKeys('(^(n))')
Sleep 2
start microsoft-edge:http://yahoo.com
The app.activate line where it says "Google - Microsoft Edge" will need to be replaced with your first websites title window text. Hovering over the edge icon at the bottom of the screen with only that website open will tell you what it is.
**Edit working version as of 8/8/2019
start microsoft-edge:http://google.com
start microsoft-edge:http://yahoo.com
$wshell = New-Object -ComObject wscript.shell;
$wshell.AppActivate('Google - Microsoft Edge')
Sleep 2
$wshell.SendKeys('(^(+(n)))')
I wonder if the command does not work twice because you're starting the process for Microsoft Edge, not just a window. Maybe two of those commands would work for IE, since each window is a separate process (afaik). Perhaps this will help? http://stackoverflow.com/questions/188850/how-to-launch-multiple-internet-explorer-windows-tabs-from-batch-file
– Kaizerwolf – 2016-06-16T13:31:27.967