You can try this and see if it works. Copy the distinct count and paste it in the same section on the report. Right-click the distinct count you copied and scroll down to Edit Summary... Then change the summary type in the 'Calculate this summary:' drop-down box from Distinct count to Count. Then go into the Cross tab expert and replace the Distinct count with the Count you just created. Sounds as if you want to use the Distinct count on another part of the Cross Tab so make sure you do not delete the original summary as your Cross Tab will still need it where it is.

I don't do a lot of cross tabs but copying the summary fields and changing the summary types works in other reports so it should work in cross tabs reports too. You may use the Report Explorer or Cross Tab Expert to make sure you have the new summary where you want it on the Cross Tab.

Hope this works for you!