Identify Your Platform
Please identify the version of MicroStation, or other product such as PowerDraft, that you are using: MicroStation CONNECT or MicroStation V8i.
The APIs supplied with MicroStation CONNECT are different to those supplied with MicroStation V8i. Consequently, our answers are likely to be different.
Code Syntax Highlighting
When you post code, use the Forum advanced editor's syntax highlighting tool. That's the icon that resembles a pencil: 
COM
Although you are writing an app. using C#, you are communicating with MicroStation via a COM InterOp. You are therefore constrained by the limitations of COM. One of those limitations is that COM cannot distinguish difference processes of a given executable. That is not a MicroStation issue, but a general problem with COM.