Determinstic ordering using a second column if the value of the first is null

[tbrammar]

Is it possible to use deterministic ordering, but supply two columns and if the value for the first is null take the second value?

For instance, to achieve this in Rails I'd normally write something like order(Arel.sql("COALESCE(column_1, column_2) ASC")). Is functionality like this achievable with Closure Tree?