Try isolation in boxes first, but within the same pen.
So, you could put them in maybe large cardboard boxes or something closed up but next to each other for a day or two. Then let them stew like that a bit, so they can feel each other is close but just not see each other. You can use food to bait them around to be initially separated also.
Then after they adjust try to move.
This isn't something I've actually done, but I'm just trying to think about how you can trick them into it. A hypothesis.
If anyone has a better idea, feel free to bring it up.