From a strictly agronomic standpoint it could be done, although depending on how much fertilizer you're putting out there, the potassium and nitrogen in the 13-13-13 could burn the seed. Our general rule for our area, which is relatively dry and fairly finely textured ground, is no more than 30 pounds total N on less than 10" spacing. Potassium, K, burns different than nitrogen, as does Sulfur. The other issues have been brought up. You must make sure you get a good homogenous mix, and that it's not sorting out as you're applying the mix to the field. It can also be corrosive to the drill if left to set. If you drill it out and then clean the drill really well the amount of damage to the drill will be minimized, just make sure it doesn't get wet while you're trying to drill. In my area, mixing 50/50 with a 13-13-13 would be too much N and K for the seed to handle, I would look for more of an 18-46-0 or 11-52-0 type product.