First off, you are NOT DEcrpyting it, you are loading the VeraCrypt driver that will open the encrypted drive and mount it. The drive remains encrypted at all times, the VC driver acts as a go between from the OS to the disk and encrypts all traffic back and forth automatically.

That being said, the easiest way is to mount the second drive manually then set it as a favorite and adjust the properties to mount it on logon. Find the favorite under FAVORITES in the menu. This is usually enough to get it to work. You may have to fiddle with the setting to mount it automatically in the SETTINGS .-> PREFERENCES dialog to make sure the VC background driver loads.