Both my BA & MPhil theses (at different universities) were marked by two examiners, who didn't have access to one another's comments or marks until after they had marked and commented upon it themselves. In fact my BA supervisor didn't know who the other marker was, or so he claims, as the marks they both submitted were in agreement and so there was no need for them to conference on it. At MPhil level, my supervisor didn't mark my thesis and I had two examiners who marked simultaneously, and then came together afterwards to make sure they agreed, which thankfully for me they did.
The external examiner's role in both cases was to make sure that marking was consistent across everyone who graded theses, and that the quality of the work and the marks awarded were consistent with standards seen at other universities: that is what their stated role in our BA handbook was, for instance. Other than that, my understanding is that the external examiner is primarily there to try & reconcile any widely divergent marks between the two original examiners. During my MPhil, a student received 2 different marks for their thesis, with the lower one meaning they wouldn't have progressed to the PhD; with so much at stake, the external was asked to assess the work and give her opinion, and so she marked the thesis with the knowledge that there was a difference of opinion between the examiners.
At both my universities, we were required to submit two copies of dissertations, in order for examiners to be able to correct simultaneously.