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On MOST cars ( I say most because I have not been inside the ECU of the BMW yet) the 2nd O2 sensor after the cat is only there to make sure the cat is doing its job. The 1st O2 sensor ensures the A/F ratio is correct. The exhaust after the cat will be different than before, thats why with a wideband O2 you want it before the cat. IF doing this does cause an issue, it will throw a CEL within a couple hundered miles and is easily reversible. I have done this on cars from 1995-2005 without any CEL or running issues.
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