Friction and Drag

A block of mass M is sliding in the positive x direction along a horizontal frictional surface with coefficient of friction k, its sides are aligned with the x, y and z axes. A second block, of mass m, is in contact (with coefficient of friction s) with the back end of the block (the side visible from the negative x direction) and starts out moving with the same velocity. There are two possible behaviours for this system: (1) The second block immediately starts sliding down. (2) The second block "rides along" with the first one until they stop, then slides down. Which condition determines when behaviour 2 occurs (behaviour 2 occurs when ________)?