Woiuld this work?
UPDATE MirrorTable SET
a = inserted.a
b = inserted.b
c = inserted.c
FROM Inserted
WHERE Inserted.id = MirrorTable.id
INSERT INTO MirrorTable (id,a,b,c,d)
SELECT id,a,b,c,d
FROM Inserted
LEFT JOIN MirrorTable ON MirrorTable.id = Inserted.id
WHERE MirrorTable.id IS NULL
Its late, im tired, so I didnt check if I did the sql right. I'm sure its close though.