it's going to rain tomorrow so i doubt that many people will rush but who knows. From what I remember people who arrived even by 8am still got tix for both parts. The line definitely got a lot longer after 9am. Tomorrow might be different though given the weather.
I don't know whether you are asking about tomorrow (Saturday) or another date in the future, but the answer is it depends. On Saturdays, the tickets are sold together whether you buy ahead of time or do rush or lottery. If you only want to see Part 1 on a Saturday, I don't think there's any way around it unless you can find someone to buy your Part 2 ticket. Or get the pair so cheap it doesn't matter that you won't use the second one, as Sondheimite suggests.
On Wednesdays, though, the tickets are sold separately. So you could buy a ticket ahead of time/rush/lotto for Part 1 no problem.