If you have multiple websites, you might ask yourself if you should create a different Dynamic Yield site for each one, or manage all of them in one Dynamic Yield site entity. This has tremendous impact on your work and how you view reports. There is not one correct answer for all cases, so use this article to decide what is best for you.
Guidelines
Do you have a single user base for both sites?
If a user can open an account in both sites, but it is a different account (e.g. different passwords per site) - you should not unify the sites.
Is it the same team working on both of the Dynamic Yield sites?
In Dynamic Yield, the permissions are defined per site. If you unify the sites, all editors of the unified sites will be able to edit both domains. Separating it will allow you to set one team as editors in site A and another team as editors for site B.
Do you use the same product catalog for both websites?
If the different website is basically a similar site but for a different locale (e.g. one for EN and another for DE), you can add an attribute for each product, stating different properties (e.g. different name, price, availability in stock) - per locale. Locale will be determined in the <head> of the account. Learn more about multilingual support.
Do you run the same campaigns across websites?
If most of the campaigns are similar, you might consider using one Dynamic Yield site. You could then target different websites when needed per experience or per campaign. When campaigns run across all websites, you will only be able to see reports that are not broken down by website. Same goes for the Audience Explorer.
Alternatively, you can keep sites separated, and when needed, copy templates, campaigns (for most of the campaign types) and evaluators. Some entities cannot be copied, and must be manually recreated in each site (recommendation strategies, recommendation campaigns, multi-touch campaigns, audiences, goals).
Summary
When possible, it is recommended to minimize the number of sites. Consider your workflow - is it just a technical different domain, but overall the same business? In that case, it is probably worth using a single or smaller number of Dynamic Yield sites. However, if your websites represent different "businesses" (e.g. a different brand), with different teams, goals and campaigns and user base - you should probably create a different Dynamic Yield site for each one.