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We propose XMM-Newton+HST observations of SPT-CLJ2228-5828 comma a high-redshift (z = 0.73) candidate for a dissociative galaxy cluster merger. It exhibits two major comma widely-separated (940 kpc) centers in the galaxy distribution with (noisy) counterparts identified in an initial weak lensing analysis of the cluster. Traced by the SPT Sunyaev Zel.dovich signature comma the hot intracluster plasma appears to be located in the middle between the two peaks of the galaxy distribution and mass map comma suggesting a violent comma dissociative merger. If confirmed comma SPT-CLJ2228-5828 would be among the widest dissociated mergers known. These systems are extremely rare comma especially at high redshifts comma and provide the best conditions for constraints on the dark matter self-interaction cross-section. The requested high-resolution X-ray observations will enable us to to efficiently confirm the dissociated merger nature. Additionally comma they will allow us to constrain the cluster X-ray temperature and luminosity to 10% comma providing the precision needed to compare its properties to its peers among the rarest violent cluster mergers known. We also request complementary HST observations to substantially improve the mass reconstruction using strong and weak gravitational lensing. This will not only allow us to compute constraints on the dark matter self-interaction from the system comma but also to compare the cluster properties to expectations from mass-observable scaling relations. Complementary GMOS multi-object spectroscopy for the cluster has already been approved comma yielding the final piece needed for a comprehensive multi-wavelength analysis of this extreme cluster.