It really doesn't matter how you mix them. It will be fine -- nothing will blow up or catch fire if you put one into your water before the other or even put them both in at the same time.
No you cannot just take the weight of one lye and divide that weight between KOH and NaOH. There are different numbers of molecules per gram for each lye, so you will end up with the wrong total amount of lye to properly saponify your fats.
There are several ways to calculate a mixed lye recipe. You found one by using the Soapee calc that allows mixed lye recipes. The Summerbeemeadow calc does the same. If you end up having to use a calc that doesn't allow mixed lyes such as SoapCalc, you can still get the job done. Here's how --
You are going to calculate the exact same soap recipe twice. The only difference between the two versions is your choice of lye -- use NaOH as the lye for the first version and KOH as the lye for the second version.
TIP: If using soapcalc, be sure to put a check mark in the box for 90% KOH purity if needed for the KOH you are using.
For the first version, the recipe calculator will tell you how much NaOH that you would need to use if you wanted NaOH as ALL of the lye. In the second version, it will tell you how much KOH you would need to use if KOH was ALL of the lye.
The last step is to calculate the weights you really need for your specific recipe. Since you want a mixture of 40% NaOH and 60% KOH for the recipe, multiply the NaOH weight times 0.40. And multiply the KOH weight times 0.60. This will give you the correct weights of each lye to mix together to make the recipe.
