We have a case where a client has an old, pretty basic umbraco website which is available in ~10 langs and we're currently migrating this to the latest Umbraco. However, they also have an extranet that's currently in another CMS entirely that they want to get rid of, so we're also migrating this to Umbraco. They don't really share any content, blocks or anything like that today, but from an editor experience point of view, would it make sense to put both of these sites in the same Umbraco installation for ease of access? E.g. have two root nodes in the Umbraco Content Tree where one site has a multitude of languages and is bound to the public web domain, and the other which probably only has English as a language and is bound to a subdomain (extranet.domain.com e.g.).

I'll try to get the conversation started 🙂 I'd be more concerned with access control, the type of information being shared, and the risk of overexposure (internet vs. extranet vs. intranet). My first inclination is to tell you to keep these separated. An extranet or intranet will typically have access restrictions–more than just "user accounts," including IP range restrictions and perhaps a different set of IT requirements (HIPPA comes to mind). (Given the company's size, the content they share, etc., you may determine if merging these is okay. This is just food for thought.) I don't know what you're building and can only speculate on what "sharing" means to you. I will inform you that there is a lovely Content Delivery API to work with to pull data out of Umbraco in JSON format. (It's not used to pull and push content from one Umbraco system to another. For that, you should look at uSync Complete.) Umbraco can handle multi-tenancy (multiple websites), along with multilingual content for each (thinking variants, but you could also have these as tenants).

+1 for keeping them separated. The benefit of the multi-site approach is that you're only developing/deploying a single app. The downside of the multi-site approach is that you're only developing/deploying a single app. I've got a number of "multi site" installs where they were stuck together to supposedly save time and money, but end up costing way more in the long run. Deployments and bugs will effect both sites at the same time - if one goes down, both do. You need to synchronise development of two sites together, which can be a pain. As features diverge over time you can end up with an overly large and complex codebase. Umbraco gets slower to boot and less efficient generally as it has to handle more content in one single instance. IME, these drawbacks far outweigh the overhead of maintaining two separate instances/codebases and asking users to edit different content on different domains (which you can make easier by cross-linking with a dashboard inside the backoffice and using SSO).

Consider whether the public web and the extranet would converge in the future. Sometimes an extranet is just a signed-in version of a web site. The ones I'm seeing lately are definitely the same "application", just with signin-gated parts. So for me, alot of same-installation web+extranet-scenarios. But if that's not gonna happen, then I'd keep two Umbraco installations. The extranet could reasonably have different user/member login providers and other Umbraco configuration which would make the one-installation approach hard.