Once I did the same but in a tricky way. You could try if you would not find any better solution.

There's a html rule Web-Login in pega which handles the login screen. You could add all your screens in that rule, but hide them but default (put them in <div style="display:none"></div> tags).

Then, create a function in Web-Login:

function get(name){
   if(name=(new RegExp('[?&]'+encodeURIComponent(name)+'=([^&]*)')).exec(location.search))
      return decodeURIComponent(name[1]);
}

Lets say, your applications is App1 and App2. So there would be 2 urls:

http://.../prweb?appName=App1

http://.../prweb?appName=App2

Using the function you will get an application name (get('appName')). And based on appName you will show specific <div> in Web-Login rule.

It's kinda workaround, but it works! :)

Better solution may be to look into web.xml and to play with different dispatchers. But im not sure it will work