Yes, AttachThreadInput ... I've already attached my app to another program...
I got the handle of the other app and used GetWindowThreadProcessID to return the necessary values for AttachThreadInput.
I know it worked because the return value of my SendMessage was 1.

My question is, how can I give my app and the other app the look of "focus" at the same time?
I know it's possible because I've seen it done.