One is enough.

Though technically it was too, I accidentally restarted the first through the in-game restart modes.

Second one I just blatantly gave myself enough money to get to the monstrous amounts of clippers so it takes only minutes to proceed to the second part.

But it's worth to see the both endings.

I replayed it last week and managed under 6 hours (2nd time). Probably some poor ordering of perks; plus I shut off the auto-bet thing to find I needed a lot more of the resource it provided. Otherwise I found this playthrough pretty straightforward, and could probably do it in a few hours now.