I need to configure my SP 2013 environments for apps, and as a part of it app domain creation is required. Can I use the same app domain for 2 different SP 2013 farms or each should have individual one? Or it all depends?
For SharePoint-Hosted apps you'll indeed need one domain per farm! Otherwise you cannot setup your DNS to direct traffic to the respective WFE servers in different farms. You could go for domains like mydomainapps1.com and mydomainapps2.com - they do not need to be descriptive. See Technet for details: https://technet.microsoft.com/en-us/library/fp161236.aspx
As I see it, the only way to use the same app domain for multiple farms, would be a smart network load balancer (NLB) that can route traffic based on different app prefixes. Would be cool and eliminate the need for more than one wildcard SSL certificate.
Provider-Hosted apps is a different story - they do not depend on the app domain.
Lars gives a great answer above, but keep in mind that if your developer (or you yourself) create a "Provider Hosted" app, and then later decide to add a list or page or whatever to your app manifest, you will automatically end up with a hybrid between "provider hosted" and "Sharepoint hosted", and you will in fact have both an "app web" and a "remote web".
Since this is likely to happen to your app as you add more and more features to it over time (lots of reasons to add a SharePoint hosted list to a Provider hosted app, e.g. to store some state), I would say you should always have a different app domain for every farm.