For now, IPTV is really limited to 4 streams at a time.
IPTV stands for Internet Protocol Television. We really have to consider that our TV programs are fed to the receivers through the internet. And as with any bandwidth capacity, the network can only hold so much.
You can compare it to your internet activities on your computer. The more you download, stream, etc, the slower everything gets.
With that being said, DVR playback does not count as a stream
Streams prioritize based on the type of stream and the remote control activity on the receiver. Stream priorities are (from highest to lowest):
On Demand/Help on Demand (HoD)/Free on Demand (FoD)
As paulgarcia pointed out, this has to do with something called "Stream Management". Live TV, Pay-Per-View, U-verse Movies, On Demand, Recording TV shows, and applications come as streams of content to the TV receivers. We provision each household with a maximum amount of network bandwidth based on the capacity of the network and the distance from the home to the serving terminal in your neighborhood. Stream management limits the number of streams so that it won't exceed the available bandwidth. Each U-verse TV customer has a total of 4 streams, they're broken down into HD (High Def) and SD (Standard Def) streams.
For your particular issue, I was able to locate your U-verse account and due to the distance of your home and the available bandwidth, you are setup with 3 HD streams and 1 SD stream (4 total). When 2 TV's are watching the same channel, they are only taking up 1 stream, if you changed the channel on one of those boxes then you would be taking 2 streams. Also, when the DVR is recording a show, it takes up 1 stream as well. So, assuming you are recording 2 HD shows while watching the same channel on 2 TV's in HD that just about comes up to the available bandwidth you have (3HD//1SD). Also like paulgarcia noted, watching recorded content on the DVR receiver does not count as a stream.
I hope this helps! Sorry if it was a little confusing, but the way the service is structured is to make sure all customers are able to utilize the right amount of bandwidth without affecting other customers' service quality in your surrounding area.
If you encounter any issues with your service or equipment, I recommend checking out our Troubleshoot & Resolve solutions to help diagnose the issue.
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.