The first app can have an iframe that calls/runs the script of the second app and passes any required variables to it.
This idea assumes you have written both apps and therefore know the app names and paths where they are installed on any given system:
So inside App1 we add this...
var F=document.createElement('iframe');
F.className='SomeStyle';
F.name='SomeName';
F.id='SomeID';
F.src='file:///C:/App2/Main.htm';
F.setAttribute('data-UserName','John Citizen');
F.setAttribute('data-UserPassword','password');
document.appendChild(F);
And inside App2 we can check to see if the program was started from inside a frame indicating that it is running as a "child process" like so:
if(window.self!==window.top) {Process_As_Child_Process();} else {Process_As_Main_Process();}
If a "child process" is true then grab and use the variables from the parent frame...
var UserName=parent.getAttribute('data-UserName');
var UserPassword=parent.getAttribute('data-UserPassword');
I hope this helps.