This can be modelled using an external tap controller or a master-slave mode.

First of all, both transformers shall have a type which includes settings for the tap changer. In addition, the load-flow options for the automatic tap changer control must be active.

For the external tap controller, a tap-controller (ElmTapctrl) have to be created and must be assigned to both transformers. This can be done in the tap-controller themselves as well as the settings for the control.

For the master-slave mode, the transformer element settings have to be defined accordingly: one of the transformers is the master, which means that it sets its tap position. The slave immediately follows, which means that it takes over the tap position of the master. The parameter which defines the controlled node in the slave shall be set to "EXT", with a reference to the tap controller of the master.

In the attached example, both approaches are shown in an own study case. For the master-slave mode, the  transformer on the left side is defined as the master, and it controls the voltage at the LV bus. The transformer on the right side is the slave.