Downloading Apps problem

I have an iPad 1. When I log into iTunes with my username and password, I can download music to my iPad. However, when I try to download any Apps updates, and I use the same username and password in iTunes, I get a response that they do not recognize my username or password. I used to be able to download from both iTunes and Apps with the same username and password, but recently I have not been able to get into Apps to download any updates. Why the disparity? Why would I need two different usernames and passwords, and how do I find a way to download Apps?
If anyone has an answer to this problem, I would appreciate it.
Thanks,
John

I can verify that you do NOT need two accounts. One iTunes account should allow you to buy music, apps, ebooks, movies, TV shows and anything else that the app store sells.

Is your username or password a mixture of cases? That might be causing the issue (ie incorrect input).

If you're sure that's not the issue, check with iTunes support.

Problem accessing Apps download

I have been very careful to make certain that my username and passwords are entered correctly. I had my wife do the same process and had the same results.
Also, I have been in contact with Apple about this problem and they promptly send me to the problem solving sites, and again I could not find anything that corresponded with my problem. I think I will just take it into a store and see if a techie can help.