In Yukon, there's a new concept called Measure Groups. A Measure group consists of the group of measures that come from a single fact table (get it? measure group...group of measures?).
Actually, measure groups are pretty cool. They let you put easily measures with different grain into the same cube. Standard example is Actual Sales stored at the day level and Budget Sales stored at the month level.
There is, however, a slight, er, inconvenience in UI in Beta 2. You can only make one measure group from a single fact table. Once you “use up” a fact table for a measure group, it no longer appears in the list of available tables. The problem is that sometimes you need to make two measure groups from the same fact table. One reason is if you have, say, budget and actual values in different fact table rows and want them to be in the cube as different measures. A bigger reason is if you have a Distinct Count measure. A critical performance key for Distinct Count measures is that each one should be in a measure group all by itself. So if you have other measures in the same fact table as a distinct count item, you need two measure groups. Oops.
They're going to fix the UI before the product ships, but in the meantime, there's a cool way to get around the problem. You open the XML representation of the cube, copy the chunk for Measure Group A, and modify the copy to create Measure Group B. The UI's list of available tables is none the wiser.
Bottom line is that the UI bug helps you learn how to take advantage of the XML definition, which, in a backwards kind of way, is a good thing.