We imagine Planet Nine as one member of a near equal mass pair that orbits a common barycentre outside both bodies. This resembles Pluto and Charon in geometry but is scaled to super sized icy worlds.
The continual mutual gravity prevents the orbits from relaxing to perfectly circular. That sustained eccentricity makes each body flex during every orbit.
The flexing dissipates energy in the interiors and becomes heat. Radiogenic decay in the rocky mantles then provides a steady baseline that adds to the tidal budget.
The combined heating can maintain warm interiors, mobile high pressure ice layers and a global ocean sandwiched between surface ice and deep high pressure ice.