My understanding is that:
Double Degree often means you get two full Bachelors degrees, which may be from two different universities, in two entirely unrelated subject areas. Often these programs are designed to be completed in 5--6 years total. I have also sometimes seen it used with reference to graduate programs. For example, a double degree Masters/PhD program, though this is rarer in my experience.
Double Major means you get a single degree from one university, but with a speciality in two related subject areas. For example, you could be a Physics/Chemistry double major. Typically this means you satisfy the core requirements of both programs, possibly using the electives of one to fill in the core of the other.
Two Bachelors means you get two full Bachelors degrees. Sometimes this is used as a synonym for a Double Degree, sometimes it is differentiated by requiring that both degrees be granted by the same institution.