The built-in activities (e.g. Running, Biking, etc.) can typically use three or more Data Fields depending on the device. For example, a vivoactive is set at a static three fields for a given activity, while a Forerunner 920XT can display a single field, or multiple fields within an activity.
Connect IQ Data Fields are unable to share data, so you would either write a single Data Field that displays multiple values (those from your hypothetical DataField A and DataField B), or write two independent Data Fields: DataField A that does your intermediate calculation, and DataField B that also does this calculation plus additional calculation. This removes the complication of dependencies between Data Fields that could potentially complicate things for both the user and the developer.
Another option is to write a full Connect IQ Application instead of a data field. This is used in lieu of a built-in activity, and provides a lot more flexibility in what can be displayed during the activity. Of course, writing an Application may be more complex than a Data Field. (You can learn more about the different app types on the developer site.)