No the two materials, sometimes three, almost always have different thicknesses. Most of my billets end up about 2.5 inches wide, 12 -15 inches long and 0.25 thick. Before drawing them out to thickness they are usually 2X2 square and variable length. Yes the light color is nickle silver. I have used copper, brass, bronze, nickle silver, and tool steel all in different combinations. I do not use steel in my mokume anymore - gives a very interesting effect but heats up in milliseconds when grinding. The copper transfers that heat very quickly right into your fingers.
I get best results from combinations of alloys that are mostly copper (similar melting points). I believe the nickle silver is approaching 80% copper as a base metal in the alloy. The silver colored dots are where the nickle silver began to melt and squished or moved around while compressing the billet. Since I am doing this in a propane forge I do not have exact heat control - just eye balling it and scratching the surface with a punch to test the level of "sweating". Very difficult to describe how you know when billet is fully heated but not overheated. If you overheat the nickle silver will splatter/spew out from between the layers of copper. An exact temperature electric oven would probably stop most of this.
If I were going to try and turn the billet I would probably change the overall shape and keep it square. Size is only limited by your equipment. For vessels I would think you would roll out the billet into thin sheet, heat and form, then solder/sweat the seam but I do not know. Glad to share any of my limited knowledge.
Good luck,
Matt