Thanks Christopher!
On this occasion I chose to clamp the two halves together tightly all the way around the perimeter. In conjunction with the engraved score, this prevented the cement from wicking underneath the two layers. Granted, as the cement has effectively created a filleted bond between the two layers, I could probably break the two layers apart by twisting. However, as this part isn’t structural in any way, this isn’t a problem!
Producing optically clear joint between layers is REALLY hard. The best results I’ve had are from shimming the two layers apart by a small amount, flowing lots of cement in between the two layers, allowing the cement to sit for a while and soften both surfaces, and then removing the shims. Be aware that if you apply pressure, you’ll see a lot of squeeze out!