Based on what I've read/researched/discussed :
1 SLOE to interview (for most programs)
2 SLOE's to be "complete" (most programs)
3 SLOE's required (minority of all programs)
So I would say 2 by 9/15 is just fine based on that.
Edit: from
@gamerEMdoc :
"
When do you need to have 2 SLOEs uploaded in order to receive a chance at a "first wave" interview?
Get the first SLOE in by mid-september. Most programs will extend interviews with one SLOE, knowing that you'll be able to get a second by mid-interview season. I'd definitely want to get a second in by the end of October, and I'd email programs that you'd already interviewed at to let them know that a second SLOE was uploaded to add to your application."