Is there a way to combine two different Co2 20 oz canisters?

No you cannot. I would get both tanks filled and switch them when one runs out. You could carry your second tank in something like this: a href="http://www.dickssportinggoods.com/product/index.jsp?productId=3790275&cid=CSE:GoogleProductSearch" rel="nofollow"http://www.dickssportinggoods.com/produc…/a