I just did this yesterday, all 4 of my boarding passes popped out when I hit the machine at the airport in the morning.

Essentially why I'm asking, since I'm only a silver: would like a way to check in at T-24 via the website or app for the return flight to snag a good E+ seat and get on the CPU list.

From the responses above it sounds like only at a kiosk or UC?

No, it can be done online, mobile, or via the phone.

Looking at my boarding passes from the subject flight (the joys of having an email retention policy that is "delete no non-spam email ever") the boarding pass email was timestamped at 10:39am and included boarding passes for both the outbound (10:38am departure) and return (1:20pm departure)

The key issue here is the time at the "destination"

If under 4 hours (domestic USA), common for a mileage run, the itin will act like a giant OW and the BPs will print as one. There is a some limit on the number of BPs that can be issued -- don't remember that limit but it is around 4-5-6. So a really complicated and many segment MR might not work.

If the break is greater than 4 hours, common for a day only meeting, the return will not be accessible until 24 hours in advance of the return departure. You will be able to get the return BPs prior to flying the outbound -- did this earlier in the month.

I've done many laps, e.g. PHL-IAH-SJC-IAH-PHL. The kiosk spits out all tickets at once. Same crew and plane for IAH-SJC-IAH - and same seat, I just tell them I need the miles. This late in the year they know about it. (I recall one flight long ago where there was a bunch of FT people sitting close together - and flying same legs - doing a MR as they were discussing changes to PQM on various carriers.)

IIRC it will bug out on the website, but will work fine on the app.

I fly a lot of day trips around Texas on UA.

When I check in on the app I get all of my BPs at once.