SQL Server Mobile Reporting Publisher - Gauges not allowing comparison value

I have been asked by my managers to assess Mobile Reporting Publisher as an alternative to SSRS. Although I have worked on SSRS for several years this is one of my first reports using Mobile Reporting Publisher.
I have created a dataset which even contains 2 fields called Metric1 and Comparison1 (both are number fields). I have added the dataset to the report and when adding a gauge I link it to the dataset. I choose the dataset as the dataset for both the Main value and the Comparison value. The main value will only let me see Metric1 in the pull down(which is fine) however the other dataset will also only let me see Metric1(it is though it can't see Comparison1). Am I doing something really stupid here or could somebody explain where I might be going wrong?