P1124 is oxygen sensing adaptation range 1 (cyls. 1-3) at enrichment limit.
P1126 is oxygen sensing adaptation range 1 (cyls. 4-6) at enrichment limit.
One or both of these codes are usually caused by an intake air leak - more air is getting into the engine than measured by the MAF. This results in a lean condition (more air than fuel) so the ECU incrementally adds fuel but reaches the limit before eliminating the lean running condition.
Alternatively, these codes can also be caused by not enough fuel, i.e., fuel pressure too low, fuel injectors contaminated, or volume supply of fuel pump too low.
As you have already figured out, all of these conditions should typically result in the same lean condition on both engine banks so one would expect both codes to be present. However, it is possible to have an air leak only on one bank so you'll want to work your way through the list of standard causes.
However, after some research, I found the link below about the AOS resulting in only the P1124 code. Might be worth checking your AOS also. Also check the seal of the oil filler cap itself while you're testing the AOS.
Let us know what you find.
Check engine light on, Code P1126 on 99 Boxster - 986 Series (Boxster, Boxster S) - RennTech.org Community