I would strongly suggest you find a WIRING DIAGRAM for your o2 sensor sand check EVERY fuse and connection....

When a car is started cold, the O2 sensors do not work- the car begins to warm up, AND the DME sends current to the O2 sensors (heater wihin the sensor) to get them up to temperature. If that circuit is damaged, you get an immediate "o2 heater circuit failure'. Each O2 sensor has 4 wires- one pair is hte acutal signal, the other pair is the heater circuit.

So check the connections first.

if you have 50k miles on them, you can go ahead and replace them- but if you just replace them and the error is there, you'll know what it is.

Also DO NOT wasted you money on OE BMW. Bosch OEM is perfect~$100.

Finally, the most important question: which bank is which? dunno. Sorry. Id replace both so it wouldnt matter to me.