A "dual lye" bar soap is typically made with an alkali solution of 5% KOH with the remainder as NaOH. My comments here are based on making soap with this type of alkali solution.
This percentage is not a weight percentage, by the way. Dual-lye percentages are based on the proportions of alkali molecules, not the proportions by weight.
Soleseife soap is typically made by adding enough sodium chloride (NaCl) in plain water to make a saturated solution. At room temperature, roughly 1 part NaCl by weight can be dissolved in 3 parts plain water. In other words, in 100g of this saturated solution, there would be about 25g NaCl and 75g water.
The solubility of potassium chloride (KCl) in water is about the same as NaCl in water, so you can dissolve roughly 1 part KCl in 3 parts water by weight.
If you make a saturated solution of KCl like this and used this solution to make soap, you'd be adding a LOT more potassium to the soap than if you use a dual-lye solution. There would be a real risk of making soap that is more like a liquid soap paste than a solid bar soap.
To mimic a 5% dual-lye alkali solution using KCl instead, you'd need to determine the amount of potassium (K) supplied by the dual-lye solution. Then translate that into the weight of KCl that would supply the same amount of potassium. Then make the solesiefe brine with that amount of KCl and enough extra NaCl to make up the difference.
It's a do-able task if you're comfortable with the chemistry and math and interested enough to try the experiment.
