Is it a problem to use 2 different websites with the same database?
If I use the same version of a website but showing it different to users by different domain and different website template but with the same backend and database, will this violate so good SEO practice? I plan to run a price comparison website with the same infrastructure behind it. The backend and the database will be the same. I will only show a different template on another domain but also to be hosted on the same server? Is this a problem?
Are you talking about addon Domains?
An addon domain is a domain which shares the same server with the primary but can be a separate site. Rather than separately developing websites, it would be better to make a 301 redirect to your site. Alternatively, you can register different domains and serve different content on each. In case the two sites have a little difference, people will believe that content on them is related. As such, it is preferable to make a permanent redirect to make them entirely different.