I will presume you're in "Dual mode" and "Record Plus" is at it's default setting.
First, if you know you want to record this game ahead of time, it helps. You simply highlight it in the guide and hit either SELECT or RECORD on your remote. By default, it will record on TV2. If you have another recording scheduled at that time or someone is watching TV2, it will create a conflict. You can then watch TNT or any other channel at 8pm while that game is recording.
If the game has already started, things are different. One solution is to run to TV2 and turn the game on and hit RECORD. Or, from TV1, change channels to the game and hit RECORD. It should ask you which TV to use. Choose TV2 and then hit OPTIONS. From the next screen, you have (4) options. The one you probably want is something like "Record remainder of event". Hit OKAY to get things started and to be sure, hit the DVR button twice. You should see the game listed with a "2" in front of it.