The TPS62851x follows the voltage applied to the SS/TR pin. A voltage ramp on SS/TR to 0.6 V ramps the output voltage according to the 0.6-V feedback voltage.
Tracking the 3.3 V of device 1, so that both rails reach their target voltage at the same time, requires a resistor divider on SS/TR of device 2 equal to the output voltage divider of device 1. The output current of 2.5 µA on the SS/TR pin causes an offset voltage on the resistor divider formed by R5 and R6. The equivalent resistance of R5 // R6 must be kept below 15 kΩ. The current from SS/TR causes a slightly higher voltage across R6 than 0.6 V, which is desired because device 2 switches to its internal reference as soon as the voltage at SS/TR is higher than 0.6 V.
In case both devices need to run in forced PWM mode, it is recommended to tie the MODE pin of device 2 to the output voltage or the power good signal of device 1, the master device. The TPS6281x does have a duty cycle limitation defined by the minimum on-time. For tracking down to low output voltages, device 2 cannot follow once the minimum duty cycle is reached. Enabling PFM mode while tracking is in progress allows the user to ramp down the output voltage close to 0 V.