I searched the Analyze help file for "combine timing" and found [ QUOTE ]
Drag and drop
When a timing file detail or graph view is opened in the Data window, QALoad Analyze provides the function to drag additional data from any group, such as counters or checkpoints, and drop it into the active view. Combine data from different groups for the same timing file or combine data from different timing files.
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Sounds like you may not be able to combine data for the default reports, but you can combine checkpoint and counter data in the details or graphs.