It seems that it is iexplorer.exe which is keeping one of them locked (looking at process explorer) - I have also noticed that the thindirect re-direct back popup window does not show when navigating away from this url. This is probaly the reason for this issue?
Also, I have noticed that whichever package is used first (url) then a sandbox is only ever created this this package - As is this sandbox is being used for both jre packages. Would this be isolation settings and should I be using full isolation here?