Slip bands (SBs), as a fundamental manifestation of plasticity, play a crucial role in governing the macroscopic mechanical behaviors of metals. Understanding SB evolution and its underlying slip system (SS) is essential. The widely adopted assumption that only one SS operates within one SB underlies many SB-based investigations. In this work, we experimentally resolved and quantitatively analyzed bi-basal slip in a Mg alloy under compression for the first time. Bi-basal slip is defined here as activation of two basal SSs within an individual SB and/or across parallel SBs in the same grain. High-resolution digital image correlation (HRDIC) was used to obtain nanoscale full-field deformation components and the displacement field, enabling quantification and tracking of the Burgers vector components for individual SBs or SB segments. Three representative scenarios of bi-basal slip were identified: Bi-basal slip across parallel SBs, uniformly distributed bi-basal slip in an individual SB, and non-uniformly distributed bi-basal slip in an individual SB. Slip transfer across grain boundaries can introduce Burgers vectors different from those nucleated in the grain interior, which was identified as an important driver for the bi-basal slip. On the other hand, bi-basal slips not associated with slip transfer were also observed, indicating activation of intragranular dislocation sources with different Burgers vectors in one SB. The potential effects of bi-basal slip on slip localization and deformation compatibility were discussed. This work provides new insights into nanoscale SB evolution and offers a quantitative analysis framework for investigating multiple SSs within an individual SB for hexagonal close-packed metals.