I have an iTunes account in the UK but I am going to spend 2 months of the year in the US. Is it possible for me to download from the US iTunes Store but keep my UK credit card and billing address?
No, it's not, you'll need a separate credit card and billing address for each country.
A work around would be to use your US address and use gift cards for purchases.
I do this, as I don't live in the US, but want access to some content only available in the US store.