Use what they sent you. Halletau is a great, very clean hop and will work very well with that lager recipe.
I might tend to use the halletau as the bittering hop (used for the longer boil) and the cascade as your flavor/aroma hop (shorter boil time). That, of course, if the recipe calls for adding hops at different times during the boil.
Otherwise, don't worry. 99% of great beer recipes call for mixing hops.