If 100 unique visitors you have come to the main (and only) page of unused domain name, you can use 301 redirect any requests to the main page of your "used" website. The traffic you perekinte but unused domain after some time, will stick to "used" as an additional mirror. If you have unused domain separately is not needed, then this is the correct option.
But if 100 unique visitors go to different pages, you don't have to glue the domain and to transfer part of the traffic. To do this, write the 301-s and redirects to the most visited pages of the "unused" domain name redirects to appropriate the meaning/topic of the page is a "used" website. And the main page as "unused" domain do not forward.
The domains are not glued you can use a 302-redirect is Oh. But this is riskybecause you can get the approval for the 302 hijack exploit.
Also note that if you have unused domain catches the traffic on the same theme (e.g., medical), and "used" the site is about something else entirely (e.g., about finances), then transferred to 100 uniques you will only spoil behavioral factors, because that will immediately close your irrelevant to their interests the website.