The deer will resolve any concerns of the soybeans choking out your brassica unless you go full rate on the beans.
Now I am not familiar with how you set a drill, but will seed size and planting depth be an issue for you? The beans will be much larger in size and technically should be planted deeper as well. Now I personally have tilled and broadcast and packed soybeans just fine, but that's not using a drill. Like I said - I don't know if it will affect you based on the equipment you are using. From the plant competition issue - the deer will resolve any chance of that happening for you within reason.