This is often the case when two tests of the same construct contain items of different types. Consequently, logit scales underlying different tests must be equated before the measures can be meaningfully compared. This situation is parallel to that in Physics when some temperatures are measured in degrees Fahrenheit, some in Celsius, and others in Kelvin. You can use this plot to make a quick estimate of the nature of the relationship between the two logit scales. If the relationship is not close to linear, the two tests may not be measuring the same thing.
Many statisticians are more familiar with the normal ogive, and prefer to work in probits.
The normal ogive and the logistic ogive are similar, and a conversion factor of 1.7 approximately aligns them.