Exception from HRESULT :0x89710016

For that behavior, you want to pass this flag on the first target:

DBGLAUNCH_CannotDebugAlone = 0x00000400 // this process cannot be debugged alone -- stop debugging when only processes with this flag remain

Assuming that the first target is attaching to a web server, you may also want this flag on the IE target:

DBGLAUNCH_WaitForAttachComplete = 0x00000040, // when DLO_LaunchByWebServer, wait for the attach to finish before continuing to launch other targets

This tells the debugger to not launch IE until after your debug engine has sent the LoadComplete event.

@Answer From Microsoft Site