How can I enter a double word keyword (say 'SEO Marketing') to a site which says 'Please enter space separated keywords?'
Just enter it as: SEO Marketing They are just telling you that spaces are ok in the keywords. For some social bookmarking websites you need to enter such keywords like seo_marketing or "SEO Marketing". Usually you get to separate keywords with a comma. So thet would know it's not 2 separate keywords.
I go with Vansterdam. You can enter the keyword as SEO Marketing. The site that says “Please enter space separated keywords†is actually saying that spaces are just fine between keywords. But there are certain sites which demand you to use keywords such as seo-marketing. Generally you need to use separate keywords with a hyphen. So that the sites understand that it is a single keyword joined by the hyphen.