  We present VLT/MUSE spectroscopy, along with archival Gemini/GMOS
spectroscopy, Magellan/Megacam imaging, and Chandra X-ray emission for SPT-CL
J0305-6225, a z=0.58 major merging galaxy cluster with a large BCG-SZ centroid
separation and a highly disturbed X-ray morphology. The galaxy density
distribution shows two main overdensities with separations of 0.144 and 0.017
arcmin to their respective BCGs. We characterize the central regions of the two
colliding structures, namely 0307-6225N and 0307-6225S, finding velocity
derived masses of $M_{200,N}=$ 2.44 $\pm$ 1.41 $\times10^{14}$ M$_\odot$ and
$M_{200,S}=$ 3.16 $\pm$ 1.88 $\times10^{14}$ M$_\odot$, with a line-of-sight
velocity difference of $|\Delta v| = 342$ km s$^{-1}$. The total dynamically
derived mass is consistent with the SZ derived mass of 7.63 h$_{70}^{-1}$ $\pm$
1.36 $\times10^{14}$ M$_\odot$. We model the merger using the Monte Carlo
Merger Analysis Code, estimating a merging angle of 36$^{+14}_{-12}$ degrees
with respect to the plane of the sky. Comparing with simulations of a merging
system with a mass ratio of 1:3, we find that the best scenario is that of an
ongoing merger that began 0.96$^{+0.31}_{-0.18}$ Gyr ago. We also characterize
the galaxy population using H$\delta$ and [OII] $\lambda 3727$ \AA \ lines. We
find that most of the emission-line galaxies belong to 0307-6225S, close to the
X-ray peak position, with a third of them corresponding to red-cluster sequence
galaxies, and the rest to blue galaxies with velocities consistent with recent
periods of accretion. Moreover, we suggest that 0307-6225S suffered a previous
merger, evidenced through the two equally bright BCGs at the center with a
velocity difference of $\sim$674 km s$^{-1}$.
