This is what I would expect. With the thermal conduction across the planar barrier being different, disparities in heat flow to the sinks and the possibility of

This is what I would expect. With the thermal conduction across the planar barrier being different, disparities in heat flow to the sinks and the possibility of one chiplet running hotter than the other parts, there might be too much stress on the joined area. This is also assuming that the package is not experiencing any additional mechanical stress from motion or vibration.