Give them the Ghost (Visible) ability. That enables them to run through each other (and other non structure units), but not buildings, yet they can still be seen.
Make a periodic trigger that goes off every 0.1 or 0.2 seconds. Then just do the action Unit - Turn Collision Off for X Unit.
if i would use this, i would make the event Unit - A unit enters region
there are things that might re-enable collision that was turned off by triggers. i have not tested, but possibly death? but for sure, (from experience) a unit with collision turned off by triggers will have collision again if it uses a skill like windwalk. (this still does not justify periodic, just detect these events, and turn off collision)
but this is solved already so no need.