Dual layer beamforming or dual stream beamforming is analogous to 2x2 SU-MIMO. E.g.:
Single layer beamforming using TM8 is equivalent to TM2
Dual layer beamforming using TM8 is equivalent to TM3
Having said this, there is some confusion when layers are referred in context of multi-user MIMO. For example an 8 layer multi-user MIMO typically means 8 different users are paired together (i.e. allocated same PRBs within a TTI) with each user having single stream only. This is the concept of multi-user pairing being used in massive MIMO.