Not sure if there's a "technical" definition for simultaneity under Thomism (I can have a look later, see if there's an article that does do that).
My understanding is that (under Thomism) at any point in time, for any effect occurring, there must be a cause currently causing it. For Thomists, effects can't just be caused by past causes. This goes for all effects, including the effect of existing. Something cannot just exist on its own, but requires something current to give it its existence. Fast forward a number of lines of argumentation, and this gets to the First Cause needing to be in order for anything to exist. Even if the universe itself is eternal, First Cause would still be required.
Basically, they're not comfortable with the idea of existential inertia,so they need simultaneous causation.
My understanding is that (under Thomism) at any point in time, for any effect occurring, there must be a cause currently causing it. For Thomists, effects can't just be caused by past causes. This goes for all effects, including the effect of existing. Something cannot just exist on its own, but requires something current to give it its existence. Fast forward a number of lines of argumentation, and this gets to the First Cause needing to be in order for anything to exist. Even if the universe itself is eternal, First Cause would still be required.
Basically, they're not comfortable with the idea of existential inertia,so they need simultaneous causation.
