This is a difficult one, as pointing 2 domains to the same website can cause problems in Google.
There are two ways of doing it:
1) As an alias, so that the website on domain1 will also appear under domain2 and the pages also appear to be on domain2
2) 301 Redirect, so when someone types in domain2 they are redirected to domain1
The problem with option 1 is to Google it looks like you have 2 websites (one on domain1 and the other on domain2), which are both identical. As a result Google will only display one site and you don't get to choose which one. This can end up causing problems with
SEO.
The second option is the best way to do it, as it tells Google that domain 1 is the one to index and is the main site.
What you're probably better off doing is setting up a separate site on domain 2, with different content so you get that indexed as well as your original content.
Alternatively you could set up a small micro site on domain 2, that redirects to domain 1 for customers to actually buy.