I understand for SU-MIMO Beamforming single layer maps to all antennas with phase shift applied to create beam. Also I understand in this case all beams will be carrying same signal (say s1). But in case of MU-MIMO how can s1 and s2 be transmitted from multiple antennas at the same time so that one of the resulting beam carries s1 and other carries s2?
Of course, a given signal and beam pattern is produced by applying that signal to the antennas, but with a certain phase. To produce multiple simultaneous signals with different beam patterns, just mix (i.e. add) the different phase+signal patterns.