We investigate the sum capacity of a fading multiple-input multiple-output (MIMO) multiple access channel (MAC) under a general class of fading, known as double-scattering. We assume the receiver has perfect channel state information (CSI), while the transmitters only have access to statistical CSI. We show that the optimum transmit directions for each user coincide with the eigenvectors of the userpsilas own transmit spatial correlation matrix. We also derive new closed-form upper bounds on the sum capacity of the MIMO-MAC under double-scattering, which we employ to obtain suboptimal power allocation policies. These policies are easy to compute, and require each user to know only their own channel statistics.