The king is seemingly programmed to target the creep with the lowest hp.
So in a team of two:
He will hit a creep and one-shot it. The splash brings the second targeted creep to half hp.
He then targets the creep at 50%, he one shots it.

This means that the damage spash does nothing. Since he will do one attack per each creep regardless in that scenario.

This forces you to make one player leave in a team of two if you wanna win.