Q: How can I acess 2 different app stores through 1 Apple ID?

When you're in a country, click here and use the instructions to choose its iTunes Store.

(123110)

You really cannot.  You cannot use other countries itunes stores. You must be within the Country with a Valid Billing Address and Credit Card for that Country to use the iTunes Store of that Country.

iTunes Store Terms of Service

http://www.apple.com/legal/itunes/us/terms.html#SERVICE

Change iTunes Store Country on an iDevice

1. Tap Settings;

2. Tap iTunes & App Stores;

3. Tap View Apple ID;

4. Enter your user name and password;

5. Tap Country/Region;

6. Tap Change Country/Region;

7. Select the region where you will be located;

8. Tap Done.

Also, see How to Change Your iTunes Store Account Location | eHow.com.