I solved this problem by using button instead of toggles.

First define two button styles before any functions:

private static GUIStyle ToggleButtonStyleNormal = null;
private static GUIStyle ToggleButtonStyleToggled = null;

Then in OnInspectorGui() make sure they are generated if null:

if ( ToggleButtonStyleNormal == null )
{
  ToggleButtonStyleNormal = "Button";
  ToggleButtonStyleToggled = new GUIStyle(ToggleButtonStyleNormal);
  ToggleButtonStyleToggled.normal.background = ToggleButtonStyleToggled.active.background;
}

And then use the idea proposed by @jzx to toggle the styles:

GUILayout.BeginHorizontal(  );

if ( GUILayout.Button( SetAmountFieldContent, _setValue ? ToggleButtonStyleToggled : ToggleButtonStyleNormal ) )
{
  _setValue = true;
  _smoothValue = false;
}

if ( GUILayout.Button( SmoothValueFieldContent, _smoothValue ? ToggleButtonStyleToggled : ToggleButtonStyleNormal ) )
{
  _smoothValue = true;
  _setValue = false;
}

GUILayout.EndHorizontal();

This produces what I wanted:

I solved this problem by using buttons instead of toggles.

First define two button styles before any functions:

private static GUIStyle ToggleButtonStyleNormal = null;
private static GUIStyle ToggleButtonStyleToggled = null;

Then in OnInspectorGui() make sure they are generated if null:

if ( ToggleButtonStyleNormal == null )
{
  ToggleButtonStyleNormal = "Button";
  ToggleButtonStyleToggled = new GUIStyle(ToggleButtonStyleNormal);
  ToggleButtonStyleToggled.normal.background = ToggleButtonStyleToggled.active.background;
}

And then use the idea proposed by @jzx to toggle the styles:

GUILayout.BeginHorizontal(  );

if ( GUILayout.Button( SetAmountFieldContent, _setValue ? ToggleButtonStyleToggled : ToggleButtonStyleNormal ) )
{
  _setValue = true;
  _smoothValue = false;
}

if ( GUILayout.Button( SmoothValueFieldContent, _smoothValue ? ToggleButtonStyleToggled : ToggleButtonStyleNormal ) )
{
  _smoothValue = true;
  _setValue = false;
}

GUILayout.EndHorizontal();

This produces what I wanted: