My last experience attempting to do this resulted in my decision to never book with expedia again. They will let you 'request' seats, but they never seem to ever forward the request to AC. I discovered this after I requested seats and then they continued to show available at AC.com. Calling AC doesn't seem to work either. All this despite the fact that you've paid for Tango + and have status.

I called Expedia and that was more than a complete waste of time.

Hope you have better luck.