I can’t say much about the musical side of this, Chick, though I have some melodicas that are tuned higher than others and higher than the instruments on my Band-in-a-Box accompaniment, which produces some interesting effects. But I have done a lot of tuning. For starters, melodicas usually come tuned closer to A=442 or 443 than to A=440. So I would begin by tuning one down to A=440. It is easier, in my opinion, to tune up since it involves scraping the tip of the reed rather than the base. Tune the other one up toward a half tone higher. But I’d suggest doing this incrementally, so that you can see what effects you’re getting with the melodicas playing together. This is an opportunity for experimentation.
Significant changes in the tuning of a melodica are more likely to be successful with some brands and models than others, less successful with thinner, more fragile reeds as on a Suzuki Study 32, more successful with stouter reeds as on a Suzuki M-32 or M-36. The upper level metal tray Suzuki models have, in my opinion, stouter more durable and predictable reeds than most other models and brands. There is also (again, in my opinion) more tolerance in adjusting their gapping. It would be cheaper, of course, to experiment with an inexpensive model.