As Wild Hearted Ponies mentioned, splash white doesn't result in lethal white. The two genes which can cause lethal white are frame and white.

Even then, breeding two frame horses together doesn't automatically guarantee a dead foal. Any foal that is born with two dominant copies of the frame gene (that would be OO) is lethal white and automatically dies. Each frame parent can only have one dominant copy of the gene, making them Oo (because otherwise they would have been lethal white). So breeding two frame horses together gives you a 25% chance of a lethal white foal, a 50% chance of a surviving frame horse (Oo), and a 25% chance of a non-frame horse.

However, breeding a frame horse (Oo) to a non-frame horse (oo) produces the same chance of getting a frame horse, without the risk of lethal white: 50% chance of a frame horse, and 50% chance of a non-frame horse.

(Everything above applies to white horses as well)