That's great. I guess it's a new feature for Exynos models. I had dual 4G from day 1 on my SD 845 model.
Mind you, when both SIM cards are on 4G mode, I would get intermittent data problems. The data would just "stop flowing." To permanently resolve this, I switched the non-data SIM back to 3G.
I wrote to Samsung and they told me to come in to a service center and since my model is from Hong Kong and I live in Canada, that ain't happenin'.