You can attach a debugger to the target program while Thread Validator is monitoring the execution trace.
Thread Validator does not use the Windows Debugger API, so there is no conflict with any debugger that uses the Windows Debugger API.
You may find that with the debugger attached from time to time access violations occur within the Thread Validator dlls (svl*.dll). This is most likely when Thread Validator is trying to access some memory within a section of code guarded with structured exception handling. The debugger will always be notified about these exceptions, even though Thread Validator's code is handling them correctly. The only thing you can do is disable the notification for these exceptions in the debugger settings.