The first one you run bitcoind. It should do nothing. You should not see anything, it should just look like it hangs. Don't worry, nothing is wrong, that is what is supposed to happen. If there is any output, that is usually a problem.
The second terminal you use bitcoin-cli to interact with the bitcoind you started in the other terminal. This one is where you do all of your commands.
You should be able to use the same terminal. Just kick off bitcoind so that it will run in the background and your shell will give you the prompt back. Ie, run:
$ bitcoind &
Instead of just
$ bitcoind