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.