The two plants do look quite different.
M. haageana is a smallish plant, usually growing cylindrical in time, and its radial spines tend to be short and with just one or two centrals which can be white but normally are brown. The axils are often woolly but don't have bristles.
M. geminispina is a larger plant, forming masses of heads in due course, and tends to have flatter bodies, and the spines, both radial and centrals are usually white, with the centrals often being more numerous (up to 6) and sometimes tipped brown. The axils usually have white bristles as long as the tubercles.
When plants are young though, not all these features are apparent, so it can be difficult to separate a number of species. Counting spines isn't a secure way of identification, as the formal descriptions in the books are based on usually one specific population, sometimes just a few plants, and don't get updated as more is known over time about the variability of species.