Thank you so much everyone!!!! I'm sorry that I couldn't get back here to provide an update before now.
So, a thorough physical exam of Juju did not indicate any neurological problems (responses all normal, wing extension good, flight good, grip with feet all good), palpation of his abdomen did not reveal any growths. Juju did not ever do the wing shudder action while he was with them. They were kept in a central location and observed all day by the techs and the vet. If it were a toxin, Juju's condition would not have been something he could hide. No bloodwork or x-rays were taken today.
My vet then asked me if he was doing anything with his tail at the time of his wing shuddering. He was not. Just standing straight up, quite still, holding his wings out and shuddering them very quickly. His current working theory is--it is hormonal. Which seems as likely as anything else. Prior to Kuiper's arrival, Jelly and Juju would do the sexy budgie dance, but never fed one another and Jelly never preened Juju. Once Kuiper arrived, Juju became infatuated and fed him and preened him while mostly ignoring Jelly. It has been 8 months since Kuiper passed, and in the last 3 months, both Jelly and Juju have been displaying more "adult" budgie behaviors (practicing regurgitating to parts of the cage, etc.). They now feed each other every day, and Jelly will actually preen Juju, too.
So, previously my boys were rather inept at typical budgie-ish behaviors. Perhaps what I saw was Juju's inept way of 'having his way' with his perch?
I'm keeping a close eye on both of them. Continuing to increase their dark time over night, and will try to video the behavior for their vet if it happens again. They were happy to be home and have a nice fly around the kitchen. They ate their dinner and were happy to be covered for the night last night. I'm looking at them now on the camera, and all seems well.