There are good sides to both but many theses days use double openers just because seed placement is actually better with a double opener. What I mean by this is the seed is placed in such a way that the soil actually has less air spaces and better seed to soil contact. For food plots it really does not make much difference as both will perform equally well as long as you use the drill in the way and conditions it was designed to be used. As far as single or double, it comes down to your preference of what you do prior to planting.
As far as down pressure, that is a function of each model of drill. The GP I use uses spring pressure and can run around 450 pounds of down pressure and it has double disk openers. Pressure can be adjusted as well as seeding depth but the most important concept is going to be how much pressure is applied to the closing wheels.
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