This dual-PMIC solution is able to provide more rails and flexibility than the single-PMIC solution. The additional rails increase functionality such as allowing interface with additional ICs like TI's DP83867 Gigabyte PHYs.
TPS6521855 comes pre-programmed for interface with LPDDR4 memory. However, it maintains the programmable DIY functionality of the TPS65218xx family, so it can be reprogrammed for DDR4 memory use. Also, as further explained in Section 6, the DIY PMIC TPS6521815 can also be programmed to power this configuration.
Please refer to the Can the LP873364 be Used in a Dual PMIC Configuration? section of the Powering the AM64x with the LP8733xx PMIC application brief for more information on implementing this solution.