I have a client who runs two closely related businessess from the same location.
It is unlikely he will have much content for these at the moment but plans that his website(s) will grow in time.
Is it better from an SEO point of view to have two really small sites (about 2-3 pages each) or one larger site so that it gets noticed in Google, and then split the sites into two later?

I'd suggest you start with two completely different sites right from the get go. When I mean different, I am talking about two web sies with unique designs and content. The search engine seems to like web sites that evolve over time more so than those that are static. Make sure, if possible, that the two sites are tightly inter-linked so as to reinforce each other's off-site ranking influences. It'll take time for your web pages to generate decent rankings but they will come with continued growth.

I echo canadafred's opinion on having two separate very niche sites. As long as they link to each other, and you have a handful of backlinks going to each one of them, I can't see the disadvantage.

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