My man, either of those 600’s should be plenty by themselves. I realize you’re running them at reduced output, but a single 600 is like perfect for 1mx1m. That’s like 120 hid Watts per foot. To be completely honest, I’m not sure how you’re keeping them as cool as you are.
If it were me, I would run the mh at full power in center of that tent. Probably about 20” above the tops of your plants. Or at least above the average height of your plants. You can go lower if plants are hardened properly, but I would guess that 18-20” is going to be about right for you vegging with a 600. Place your hand palm down over your plants, if it’s too hot for hand then it’s too hot for your plants. Thats general rule, not the law.
When you decide to flower, just switch out the mh for the hps. You may want to start it a couple inches higher over canopy as the hps is generally a little higher output. As the plants stretch you can adjust lamp height so it’s a little closer to the plants. Just do it little by little over time, and you should be good. I could get 400 pretty darn close without frying plants this way, but the footprint got too small. I would guess 16” above canopy is probably as low as you would ever want light.
I believe what you’re seeing is reaction to vapor pressure density. I can’t explain it the way it should be, too scientific for me lol. But if you take the swings out of temperatures, you should see everything become more manageable.
If you haven’t already, you can look into remote ballasts and cool tubes to help with temps too. But I really think you’ll be fine with one fixture. Just keep fresh bulbs in every 2-3 cycles.