Best way is to give a clearance limit to the lower aircraft (ABC123), calculate the lat sep based on holding at that point. Give other aircraft a requirement to reach a level above by lat sep point. Trick is to make the clearance limit at a point that ABC123 will never have to hold. i.e. XYZ456 reaches the lat sep point before ABC123 reaches clearance limit, at which time you can cancel the clearance limit.
I know a lot of places don't teach use of clearance limit (and calculation of lat sep for a clearance limit), so the next best choice is use of diversion radial as per Plazbot's post.