I seem to be able to figure it out by plugging my monitor to the 7970. If i plug the monitor to R9 270, the GPU 0 (HD 7970) shows 1.5 Mhash while the GPU 1 (R9 270) shows 800 Khash. But if I plug the monitor to the 7970 then unplug it then plug it to the R9 270 I get 1.5 Mhash on GPU 0 (HD 7970) and 2.7 Mhash on GPU 1 (R9 270). Doesn't a HD 7970 supposed to have a higher hashrate than a R9 270 ?

NB : I want to use the R9 270 to play games when I want to while the 7970 is mining and use both to mine when I don't want to play games.