Any given tablix can only be associated with one dataset at a time, but there are two ways to accomplish your goal that I can think of.
First, you could create two identical tables side-by-side that are designed to pull related data from each dataset together in the same order. When you run the report, if the data are synchronous between the two tables, you can visually confirm that there are no errors. If they appear out of sync, there is a problem to investigate.
Second, you can use the LookupSet function to pull related data from a secondary dataset into the context of a tablix associated with a the dataset. By using a Split function and by concatenating a vbCRLF as a delimiter for the string created by the Split function, you can create a set of "rows" that can mimic a table within a table.