A Substitute For UNION?

I'd like to change query:

SELECT DM.*, 'condition1', NULL FROM DM
WHERE (condition1)
UNION
SELECT DM.*, NULL, 'condition2' FROM DM
WHERE (condition2)

to one SELECT like this

SELECT DM.*, WasCondition1, WasCondition2 FROM DM
WHERE (condition1) or (condition2)

but how to fill in the WasConditionX column?
The UNION version was bad because in case a row fulfilled 2
conditions, it was repeated instead of joining them, such like this:

SELECT DM.*, 'condition1', 'condition2' FROM DM
WHERE condition1 AND condition2
UNION
SELECT DM.*, 'condition1', NULL FROM DM
WHERE condition1 AND NOT condition2
UNION
SELECT DM.*, NULL, 'condition2' FROM DM
WHERE NOT condition1 AND condition2

or (say DM has columns A, B, C):

SELECT A, B, C, SUM(cond1), SUM(cond2) FROM
(
SELECT DM.*, 1 AS cond1, 0 AS cond2 FROM DM
WHERE condition1
UNION
SELECT DM.*, 0 AS cond1, 1 AS cond2 FROM DM
WHERE condition2
) AS DM
GROUP BY A, B, C