Players report that you can combine white patterns in the standalone horse creator. Breeding in the main game follows inheritance, and players have not found an equivalent way to layer two white patterns on a foal. Use the breeding lab to see possible results before choosing parents.
Layer markings in the standalone creator
Use the creator when your goal is to design an appearance and see how patterns look together. A player pointed out that this tool can combine white patterns even though they could not reproduce the combination in the main game.
Use parents' markings to plan a foal
The official Steam description says foals can inherit a mix of coloring, white patterns and personality. Different traits can come from each parent. A pair of parents with distinct patterns is not evidence that their two patterns will stack.
Compare actual previews
If you want a specific pattern, choose a parent that shows it and compare the available lab previews. Try another pairing if the first one offers no result close to your goal.
FAQ
Can I combine white patterns in the creator?
Players report that the standalone creator supports this.
Can I breed a foal with two stacked patterns?
Player reports have not confirmed an equivalent breeding option.
Where can I check possible markings before breeding?
Use the advanced breeding lab to preview possible foals from the pairing.