Yes, you can't run multiple instances of a node using multichaind for the same blockchain at the same time on the same computer, unless you do special things to separate the directories.
In the second terminal window you usually want to run multichain-cli, which talks to the node to perform the functions you want. Try running something like this:
multichain-cli first getinfo
Then follow the Getting Started guide for more ideas:
https://www.multichain.com/getting-started/
