I have always mixed them 50/50 by volume, and have had no issues.
How much you mix up depends on how much you need. Is it a bow with 6 lams or 3? A hickory backed bow or a limb for a takedown?
For a 6 lam longbow, I mix 3 oz. of each, which together fills a tuna fish can... which I have the wife save for me.