Making a solution with two solutes I need to make a solution that has a 15% (wt/wt%) HCl acid concentration and a 6% (wt/wt%) NaI concentration. I'm not sure yet how much of this solution I will be required to make so a general recipie would be awesome.  I would also like to know how to make the concentrations wt/vol% as well.

Additional information:

I have a bottle of Muriatic Acid. Its label says the contents are HCl acid, 20 degree Baume (31.45% by weight). The product's MSDS sheet says its 65-75% water and 25-35% HCl, and its specific gravity is 1.16 (1.16 g/ml).  

I have Sodium Iodide (a brick of it). Its label says 99+%, pure, anhydrous.
You need 15 g HCl and 6 g NaI per 100 g of the solution. Treating 31.45% (I don't know much about practice) as exact, there is 15 g HCl in 15 g / 31.45% = 47.69 g of your muriatic acid. So you need 6 g of sodium iodide, 47.69 g of your muriatic acid and the rest, i.e. 46.31 g, of pure water per 100 g of the solution.