Sampling from a couple of positively correlated beta variates

We know that the marginals in a Dirichlet distribution are beta variates exhibiting a negative correlation. But we can construct two linear combinations of such marginals in such a way to obtain a positive correlation. We discuss the restrictions that are to be imposed on the parameters to accomplish such a result. In the case the sampling from the Dirichlet distribution is performed through a generalization of Johnk’s method we discuss the efficiency of the algorithm implementing the method.