Which container to use for combined milk? My local supermarket was out of 1% milk today, so I decided to get half a gallon of 2% and half a gallon of skim milk and combine the two at home. Now I'm wondering in what I should store the combined milk...

I still have a 1 gallon container from the last milk I purchased which I can use. There's a tiny bit of milk left in there, but it will be finished by today. The only thing I'm worried about is that, even if I wash the container the best I can, it will still have traces of bacteria in there from the old milk. Does this mean, then, that I should treat the expiration date of the combined milk as the same date printed on the old container?

Should I store the milk in some other container (e.g. a pitcher)? Should I temporarily combine the two in a pitcher, then put them back in the two half gallon containers they came with? Any other ideas?
Don't reuse an old milk jug for storing the new, combined milk. The standard plastics used in milk jugs in most areas cannot be properly sanitized for reuse (even with standard sanitizing solutions). 

Instead, use a properly sanitized and covered glass or plastic container, preferably one that you clean in a hot water dish washer.  Milk stored in a properly cleaned container will have the maximum shelf life (and reduced chance of other contaminants).