One roku can run one app. So to run two apps you either need to use two rokus, or you need to get the channel code of what you want to overlay and merge it into your existing main app to display on top of it's regular screens. So yes. I am sure. Nobody has NetFlix with a Twitter app running on top of it.
I suppose another option would be to use two rokus and try to use Picture-in-picture on your TV to display HDMI port2 in the corner while HDMI1 is using the rest of the screen? But that's not your question - your question was can you run two apps at the same time on the same single Roku, and no, you can't.