For my example batch, I poured the ciaglia first with a cardboard divider already wedged into the corner of the mold, and then as I poured the ciaglia portion I leaned the cardboard over the top to the opposite corner until the ciaglia was encapsulated into a triangle shape. I left it sitting in the mold with the cardboard on top while I mixed up the plain part. When that was ready to pour I dragged the cardboard divider up the diagonal, scraping it against the top edge of the mold to ensure a slick edge. Gave it a quick press over the top with a flat knife to ensure it was straight, and then poured the plain into the triangular shaped cavity left in the mold. I preferred the ciaglia first because I was using an accelerating FO with the ciaglia so i knew I would get a crisp edge to put the plain soap directly onto in the same soaping session.
Can I just say I'm well jel of how fine some of you can get your ciaglia 'lumps'. I can't seem to get mine as fine as that, no matter how much I stick blend!