My first supervisor is the one who takes the lead, but my second supervisor is just as involved - he's less experienced at supervising PhD students so is learning the job of supervisor from my first supervisor as he goes along. They both attend all the meetings (unless they're at a conference or something) and comment on papers I write or on the methods I've been developing, etc, though tend to defer to each other's specialist areas as their disciplines are very different. We tend to meet every 2 or 3 weeks, depending on what there is to discuss, usually for between half an hour and an hour. It works really well - my first supervisor is very detail-oriented and highly organised, so he drives things forward, while my second supervisor is quieter but very thoughtful so tends to sit back and then come in with brilliant observations or new directions or sharp questions.