Original flight from ORD to MSN cancelled due to incoming weather. To standby for earlier flight, had to split the reservation I was on with my daughter, so I'm #1 on the standby list, she's #12.

So it appears for standing by for a flight, after a cancellation, you're forced to split the reservation, and the lesser-status person ain't gonna make it? The agent said something to the effect that they'll spot two people with the same last name and try to accommodate, but not sure if that means they'll just make sure that we're not split up (I get on and she doesn't... so neither gets on). Definitely don't want to leave her behind.
Updated after flight-
Different agent now working the gate. 27 people on standby list, I'm #1, my daughter #12. At first it looks like only one spot on the plane, which I decline, explaining that I didn't want to leave her behind. She surmises that it's a split record and my daughter was on my original PNR, says to hold on, does a physical check of the seating on the plane, a second person hasn't shown up so she gets us both on. I don't know if this is by the book or not, but it was certainly appreciated.

From what I heard shortly afterward, UA3705 was the last flight to leave ORD for a while. The thunderstorm was moving in right after we boarded, and I was wondering if they were going to shut down ground operations due to concerns about lightning strikes. It didn't look very friendly out there.