Molar solution needs to be more concentrated?

I have a 100mL of .1M HCl, it's not reacting well with Zn and Sn. I want to increase the molarity and see if that works better. I have 2M stock. Is there a way to calculate how much 2M HCl to add to my .1M HCl? I'm thinking a .5M solution would be better?