In N. America, the incoming 220 volts are split into (2) 110 volt legs in you main panel box and they are in opposite phase. Each of these legs also has other appliances (breakers) on them which add noise to the leg. If one of your circuts that feed your equipment is hooked up to one leg and the other equipment circut is hooked up to the other leg, then you are picking up the noise on both legs and raising your noise floor. If you hook up both circuits to the same leg in the panel box, you are only getting half the noise, because you are only picking up the noise on that LEG as opposed to picking up the noise from both legs.
In addition, you can avoid any noisey appliance breakers ( Like A/C, washing machine, fridge and furnace breakers) to the other leg that your equipment is not on, further reducing the noise on the leg that you are using for your equipment.
In addition, you can avoid any noisey appliance breakers ( Like A/C, washing machine, fridge and furnace breakers) to the other leg that your equipment is not on, further reducing the noise on the leg that you are using for your equipment.