OK, I figured it out.
It's not possible to do this in one query, but inside a stored procedure it's trivial.
Firstly, I needed to know the root node (it had nothing to do with combining two queries).
To find the root node I run:
SELECT
contact
FROM
cont_reln
WHERE
relation_type = 'CH'
START WITH related_to = '?'
CONNECT BY NOCYCLE
PRIOR contact = related_to
AND PRIOR related_to != related_to
ORDER SIBLINGS BY
contact
This is in a foreach, and by virtual of the way the hierarchical query returns, I always know the last record is the root.
From there, I just run the child select based on the root contact id.
SELECT
contact,
related_to
FROM
cont_reln
WHERE
relation_type = 'CH'
START WITH contact = 'THE RETURNED ROOT NODE ID'
CONNECT BY NOCYCLE PRIOR related_to = contact
ORDER SIBLINGS by
related_to