<i>Is it realistic to visit the Falls in the morning and then go to Mt Rainier in the afternoon returning in the evening?</i>

It's doable. Obviously it will depend on time of year (daylight) and how much time you want to spend at the falls and mountain.

I would aim for the Crystal Mountain/Sunrise side first, then around the bottom to the Paradise side. Two hours seems optimistic to Paradise, about right to Sunrise.