i was also having problems mining with 2 cards only getting the second one to work properly , and the other one going slow or not working at all.
The fix was to set -g 2 to -g 1 for me ( had it copied from cgminer so i didnt notice untill today )

getting 7.5Mh/s with a 290x
and     7.2Mh/s with my 290 cards

my settings are if anyone wants to try :
-d 0 -g 1  -w 128 -v 1 -I 20 -l 1 --thread-concurrency 27400

setting a different -w value does not affect the performance for me