There is a slight disconnect with session parameters.
if you set it Server side, the client side mashup won't know about it until a reload.
If the mashups are both open at the same time, the best way to push the value into the session is to do it Client side.
So in the login mashup, have the user login, then return the User name to the login mashup and bind it to the session in the mashup vs. setting it server side.
With 7.2 btw we do have Push capability, so now you can push the value to the other mashup if so desired.