The oil burner flue should be connected above the solid fuel appliance in a common chimney. There are two reasons for this. One is to keep the solid fuel heater's potentially hotter stovepipe farther away from the ceiling. Two, is that if creosote and ashes accumulate in the bottom of the chimney, the solid fuel unit stovepipe would be blocked first. A blockage in the solid fuel appliance is mostly an annoyance, not a safety hazard. The fire would be lazy or go out. The blockage of a gas or oil appliance is a serious hazard. It will try to vent into the house CO gases, which are deadly. You could either leave it as is, since it sounds like it's performing satisfactory, or you could install one barometric draft regulator into the solid fuel unit also.
In the case you have both flues connected with a T or Y fitting before they reach the chimney, you still have the option of installing one large barometric draft regulator before the chimney after the connection of the two appliances, or two the stovepipe flue sizes in each appliances flue before the joining connection.
As for .04 inches being too low, that is is low as I'd go for a solid fuel unit. The higher the setting the hotter the solid fuel unit will fire. I would not go above .08 inches. With one barometric draft regulator set too high, performance of your oil burner maybe more inefficient do to more heat going up the chimney. Check out the oil burners draft recommendations to be sure. If it's working good for you now and your happy with the performance of each unit, I'd leave it alone. If your not, you have some options to play with.

DOUG