Ssrs using several datasets

In a new SSRS 2008 report, I will be placing 6 different existing rdls into the one existing new rdl. Each of the existing rdls have their own dataset. All of the existing rdls already have their own dataset. All 6 of the current rdls use the same parameters. The only difference in this rdl is that I will have a new parameter to allow the users to select which report of the 6 reports that they would like to have generated for them. The default will be for all 6 reports to be selected. I am not planning to use subreports.

i am planning to have one main tablix where there will be one control group called customerID. I am then planning to place 1 tablix from each of the existing reports into the main tablix sequentially where there is a control group by CustomerID.

Right now this works fine for the first report only. The problem occurs when I place the second tablix into the 'main' tablix. The second tablix only allows the dataset used in the main tablix (called Maindata) to be used. In this second embedded tablix, I can see the list of all the datasets that are available and I try to select a different dataset that is called seconddata. However after I select seconddata dataset only Maindata dataset is selected.

Thus to solve this problem, I would like to know what I should do. Do you think I should:

  1. Put all the elements that I need for all 6 reports into the dataset called Maindata.
  2. Should I use a lookup function? If so, can you show me what the syntax would look like for using the 6 different datasets?
  3. Can you tell me what other suggestions that you have to solve my problem and show me and/or point me to references that will tell me how to solve my problem?