Ordering By One Field Or Another

Is it possible without using CASE statements, to order a result set by one field (if the second field return value is null) or another (if the second field return value is not null)

This would be on datetime fields.
Let's say I have 5 records with 2 date fields
sched_dt and arriv_dt.

sched_dt will always have a value, arriv_dt may or may not have a value.

In a single result set I would like to have the records with an arriv_dt sorted by arriv_dt and the ones without an arriv_dt sorted by sched_dt.

*Hope that makes sense*

Thanks