That would be the expected result, two engines in combination, logically would detect more than one, but rather depends on actually what they have in their engines when combined and as I said what is the user dependant.
If you analyse the pdf results page 6 GData has no user defined (Yellow) or blocked on/after execution (Light Green) sections in the chart like avast or bitdefender, so you would think that these two in combination would produce a higher score, but in this test it doesn't.
GData just has, detected on scan and not blocked; to me implies it can't possibly have all of the features/function of the individual AVs in is combined engine.