Run & Apply fails if done to multiple tests at once

Maybe I'm doing something wrong, but if I do Run & Apply on `testA.kt`, and before that finishes, do the same to `testB.kt`, I get errors.
Is there a different way of running them in parallel? I like being able to do apply diffs quickly, especially when updating compiler versions

Reproduce the failure by starting Run & Apply for `testA.kt` and then `testB.kt` before the first operation finishes, recording the resulting errors. Trace the Run & Apply entry point and its concurrent-operation handling; done means parallel runs complete without errors or the supported workflow is clearly documented.

Written by the indexing model from the issue text.