Hi there, I kind of answered your question here. The simple truth is that we really don't know, but that a full-on legionary testudo would probably be unlikely. As opposed to the Roman scutum-wielding formations, an Anglo-Saxon shieldwall worked by overlapping its round shields for increased strength and protection, but of course this does leave gaps. We know from contemporary sources and archaeology that missile combat was a key part of English warfare, especially in the use of javelins to disrupt shield walls, and we have images from the Bayeux Tapestry of English soldiers raising their shields to protect themselves from Norman missiles, but this would not have been directly comparable to the more mobile Roman testudo.