  The extraordinary 2021 September-October outburst of Centaur
29P/Schwassmann-Wachmann 1 afforded an opportunity to test the composition of
primitive Kuiper disk material at high sensitivity. We conducted nearly
simultaneous multi-wavelength spectroscopic observations of
29P/Schwassmann-Wachmann 1 using iSHELL at the NASA Infrared Telescope Facility
and nFLASH at the Atacama Pathfinder EXperiment (APEX) on 2021 October 6, with
follow-up APEX/nFLASH observations on 2021 October 7 and 2022 April 3. This
coordinated campaign between near-infrared and radio wavelengths enabled us to
sample molecular emission from a wealth of coma molecules and to perform
measurements that cannot be accomplished with either wavelength alone. We
securely detected CO emission on all dates with both facilities, including
velocity-resolved spectra of the CO (J=2-1) transition with APEX/nFLASH and
multiple CO (v=1-0) rovibrational transitions with IRTF/iSHELL. We report
rotational temperatures, coma kinematics, and production rates for CO and
stringent (3-sigma) upper limits on abundance ratios relative to CO for CH4,
C2H6, CH3OH, H2CO, CS, and OCS. Our upper limits for CS/CO and OCS/CO represent
their first values in the literature for this Centaur. Upper limits for CH4,
C2H6, CH3OH, and H2CO are the most stringent reported to date, and are most
similar to values found in ultra CO-rich Oort cloud comet C/2016 R2
(PanSTARRS), which may have implications for how ices are preserved in cometary
nuclei. We demonstrate the superb synergy of coordinated radio and
near-infrared measurements, and advocate for future small body studies that
jointly leverage the capabilities of each wavelength.
