Possible to CROSS JOIN a subquery without duplicating code?
You can use a CTE:

WITH WF AS (
    select T.id, T.parentId
    from Task T
    where 
        (T.id = @tgt and T.typeId = 3)
        or
        (T.parentId = @tgt and T.typeId = 1)
)
SELECT * 
FROM WF AS WF1
CROSS JOIN WF AS WF2