Struts are developed in Servlets.Servlets are thread based.If you open two browser at a same time and are requesting same Servlets then two separate threads are created.Request attribute are thread safe.
Note that IE has a "feature" where if you open a new window via the File>New Window menu or Ctrl+N that the new window will interact with the same session. If you launch a new IE browser (say from the Windows Start menu) then it will use a different session.
WARNING! Do not activate jet boots indoors or you will see a tiny ad: