Maximizing several cuts simultaneously

Consider two graphs G_1 and G_2 on the same vertex set V and suppose that G_i has m_i edges. Then there is a bipartition of V into two classes A and B so that for both i=1,2 the number of edges between A and B in G_i is (1+o(1))m_i/2. This answers a question of Bollobas and Scott. We also prove results about partitions into more than two vertex classes.