It is recommended to have only more than one slack generators in the following cases:

Lump representation of the external network: when you have two connections from your network and another external network, it is recommended to have a lumped representation of the external network with one slack and several transformer locations connecting the two networks.

More than one external grid with only one slack: when using more than one external grid network and only one is slack, you will not experience any complications. You can set the other external networks to PV or PQ mode. The slack mode means that the voltage magnitude and angle at the bus bar are constant.

Using more than one slack: there are a some special applications where power flows in terms of different phase shifts. For example, if you study the flow of a closed ring of a distribution system you can use two external networks in SL-mode and define the phase angle of the corresponding voltages. The resulting flows will depend on the angles.