I'd say go ahead and set the two up next to each other for this grow, and observe carefully the growth of the plants relative to their positions. Do the plants directly under the metal halide grow more vigorously than those under the hps? How do they compare to the ones right in between? You can have success with either bulb; MH promotes compacter growth, HPS perhaps faster but spindlier growth, so I'd say a combination should be very effective. Whether it's more effective than two MH bulbs is something you have a great opportunity to prove.