Whenever you create a report tablix, you have to assign it a dataset, you can check that by going into tablix properties, general and check the dataset name.
Now when you already have a dataset assigned to your tablix, you can get the values from other dataset by using lookup()
Keeping the scenario in mind here, you can create a report tablix and assign dataset1 to it and then create an expression in that tablix as follows :-
lookup(Fields!ID.Value,Fields!ID.Value,Fields!Name.Value,"Dataset2")
This will lookup in Dataset2
using values of the ID column from Dataset1
and when matched, it will return Name column from Dataset2