It sounds like you have two distinct processes.
Whenever you create an instance of the TestStand engine, it is unique per process. This means that if you create two instances of the engine from the same process, it will really be the same engine. However, if you create another engine from a seperate process, it will be completely unique. This means that you will be unable to post a UI message between the two engines.
There are a couple of other solutions around this.
1) You can pass the engine from the OI to the Server somehow. (This may not be feasable for your design)
2) You can use another method to pass information back and forth between the server and the OI. Possible candidates would include DataSockets, TCP, or even the TestStand Synchronization Server (used for the Syncrhonization Step Types). The documentation for this feature is only in 3.1.
Allen P.
NI