Hello Egbert,
running multiple sites in parallel is no problem if you are using seperate site-packages. If you want to create multiple sites that are based on the same site-package neos 2.3 can do this but is currently missing the needed commands to create a new empty site based on a package. Those commands and also a be-function for that will be available soonish in 3.0.
Adding multiple domains to a site is no problem at all and can be done in the sites-module. However binding language dimensions to a domain is not that simple especially if you want to avoid the language-uri-segment wich is used by default. There are solutions for that but they have to be configured individually.
A thing that still is a bit hard is sharing/reusing contents between sites and linking between sites. Usually all those problems can be solved but it requires some experimenting to find a good solution.
If you describe the intended setup a bit more detailed i can say more.
Regards, Martin