I love Roku...
I too have had it for about 3 years. Last night I wanted to watch a streaming movie. When I fired up it would not let me on because my wife was watching one on another tv.
This is the first time this has happened. Has somthing changed?
If you pay for 1 DVD at a time service you get 1 streaming video at a time. If you pay for 2 DVDs at a time you are allowed 2 streaming videos at the same time and so on.
Your Internet connection also has limited bandwidth and may not be able support multiple streaming videos at the same time, particularly if one or both are HD.