More than one O2 sensor going bad at a time is highly UNLIKELY unless you know of a specific reason for it's occurrence.
The most important thing to remember when getting an O2 sensor CEL code is it means (and only means) the value being reported is out of range for circuit that particular O2 sensor is on. Period. It could be b/c sensor is bad / or fouled, cat is bad (if S2), or wiring at sensor is bad, wiring before sensor is bad, MAF bad (if multiple codes for each sensor), etc., etc.
Use a multi-meter to test the sensor itself before replacing. If fine, then start looking upstream from there starting with the wiring.
And finally, Bank 2 on a Boxster is on left side when seated facing forward. S1 is pre-cat, S2 post.
