In one iPad, Is it possible to download an app and susbcribe to its content with an apple id linked to one country and then use it while logged with an apple id linked to anohter country?
Yes - in general, you are allowed to mix apps from more than one Apple ID on the same device as long as the other terms of the license agreement are upheld. Technically, there is only the requirement that you first log into the account A and accept the license terms, download the apps that Account A has purchased.
Then you switch to Account B to accept the license terms, download the apps from Account B.
When you have updates, you will have to enter the passwords and switch back and forth to accomplish the updates, so it adds work and time, but isn't something I have seen blocked as a general case.
I can't speak for whether this is legal or might be a violation of the terms of service - I could see instances where if the only reason for this was to bypass restrictions in a country you could get called out or have your accounts terminated if you were not providing true information. As long as everything was legitimate in each account as it stood alone, I see no reason why you couldn't have this situation for some apps on a limited number of devices that you "own or control" according to the iTunes store terms that govern each account respectively.