However, in practice, where a party had two members, let's say, from a constituency, both in the campaigning and in the areas where they win support you would normally see that each candidate would win a lot more support around the area in which they lived than they would in other parts of the constituency. To some degree, proportional representation works in the constituency, because different areas get their own MP.
Of course, an MP isn't just an MP for a part of the constituency. An MP is an MP for the whole constituency. But typically there's one much closer to you than the others are.