Having separte Apple accounts on same device I have couple of questions to ask related to one topic.

I live in japan and had my apple account installed there. 

And now I am here in US for a year and want to make another apple id so i could redeem iTunes gift card here in US. Is it possible to have another apple id on one iPhone?

When I get back in Japan (for example, I downloaded apps using my US iTunes account) and logged in on my Japanese iTunes account, will the apps I downloaded in US iTunes disappear on my phone?
When you change countries, you will no longer receive app update notifications on your iOS device or in iTunes for the apps you purchased in your original country. 

However, you will still be able to download those applications again for free if they are present on your new country's store (even if they are updated). 

You will not be charged and the download will not appear on your Purchased page. 

If the app is not available in your new country, you will not be able to download it again.