Right now you can only have one trace visible in either profiler. Whenever you record a new trace or load a trace file, the old trace gets dropped. It would be very nice if we could have multiple traces open at the same time, and for example compare the flame graphs of two traces to see if a code change did anything to the performance numbers. Right now, in order to compare two traces, you have to either open two instances of Qt Creator or note down your findings from a previous trace and then manually check your notes against the new trace. This is very annoying.
On top of this, the fact that we can only have one trace has lead to numerous hacks and inconsistencies in the code because the "run" actions are actually designed to run multiple instances in parallel. We maintain flags in each profiler to disable the "start" button when it's inconvenient to drop a trace. As this inconvenience can take multiple forms, this is brittle. If we could have multiple traces all this mess could go away.