Certified GKH Gardening Expert
It would be a race to the finish for them as the Horse Chestnut is also alleopathic (poisonous to other plants). Typically, the alleopathy of these plants can be greatly reduced by making sure that their leaves and fruits are removed from the ground in the fall. This typically reduces the area of alleopathy to just a few feet around the trunk. If you feel you can be diligent about that, then these two trees can be planted close to each other.