I am using Dnn 4.08.04 Version

I had two separate dnn Instance in my Server ( D1 and D2 )

 I am  browsing two instace  at a time , If I signed in with user ( U1 ) into D1 instance and

with user ( U2 ) to D2   instance  then  U1 gets logged off from  D1 instance.

Both Dnn  instance Host settings was

  Authenticated Cacheability : ServerAndNoCache

 Page State persistence      : Page

Thanks to all for your help

Thanks Chris

yes two instance names were same but my first instance is child and second insance is parent 

My first Instance name is ( Ext IP ) /Parent/Dnn

My Secont Instance name is ( Ext IP )/Dnn

Even parent ( D2 ) and child name ( D1)  same will also cuse the issue .

If so Please let me know the solution for this issue

Thanks Chris

As I stated Previously

D1 -- http:/ExtIP/Parent/DNN

D2 -- http:/ExtIP/DNN

  Is there possible to change domain name Virtually , so that it may work .

I tried following steps for D2 instance , but it is not working

1 I created Virtual Directory with new Unique name ( ex : DnnNew ) for D2 Site

2 Adden new Http Alias to the D2 site ( ExtIP/DnnNew )

3 So now I am accesing my two sites   with   http:/ExtIP/DnnNew and  http://ExtIp/Parent/DNN  again same issue follows