At the moment where do you want to go? Since in Berlin half of the S-Bahn-tracks don't work at the moment, just going into the city and see there how to get from the city to your hotel can need more time than usual.
If you want to meet with someone who arrives at TXL, you should take a RE to the main station while the other person should take the bus txl (yes that's the right number) which stops in front of the main station.
With the "normal" ticket in Berlin, you can use every mode of transportation, as often as you want during 2h. You can change as much as you want.
cheers
757DUD