SSRS Report joining data sets in BID 2008
Hi I am afraid that this is not possible in 2008. You can workaround it though by implementing the logic in the dataset itself or you can create sub-reports to achieve what you are after. 

However, if you have the ability to use a 2008R2 instance you can indeed do what you are asking by using the Lookup functions. Here is more information on it:

LOOKUP:

http://msdn.microsoft.com/en-us/library/ee210531.aspx

LOOKUPSET:

http://msdn.microsoft.com/en-us/library/ee240819.aspx

I hope this helps you.