Why do you want to enable either for singleplayer? Simultaneous mode only makes human players take their turns simultaneously, while dynamic mode again only applies to how human players take their turn. If you're playing singleplayer, you're the only human player, so neither of those modes would do anything.
If you use simultaneous or dynamic in multiplayer with only AI and City States you will notice that that isn't true.
Also I would use Dynamic to prevent problems with warfare.
Try it out. I already did test it.
I mean, as the single Player in a Multiplayer game with only AI and City States it works.
Much like it did in earlier versions of the game.
City states are especially positively affected by this since in the original game it takes considerably more time between switching of command for the engine than it would if it didn't switch in the manner it does.
anyhow, just try it out yourself
put visibility of your starting unit to 25 or 35 and see the difference on first turn
without there's settlers everywhere
with there's cities and some units still moving if you're fast enough at using cheat engine to give you that visibiity to check it within the first turn
Also why would I wanna use it, perhaps because I would be fine that the AI moves while I waste ten minutes on which card to put into policies or where to send my settler.