The frogs side of things is dealt with pretty well on this site. The substrate choice for the salamander is a different matter. To give you a good answer for that one, could you please tell us what kind of salamander it is? If it's a burrowing salamander, like a tiger salamander or a spotted salamander, stay away from coconut fiber unless you're prepared to change it often. You'll have better look with a top soil mix sold at the hardware store. Just make sure it's free of artificial additives like fertilizers and perlite.