Average CO(1−0)-normalized SLEDs (in L_⊙). Colored circles and diamonds are measurements obtained in this work for systems PJ231−20 and PJ308−21. For the companion galaxies we show the two SLEDs obtained assuming T_dust = 47 K (filled half-top symbols) and 35 K (filled half-bottom symbols). Empty symbols refer to the CO ladders of other high-z quasars and one SMG (HFLS 3) from the literature (see Riechers et al. 2013; Venemans et al. 2017c; Carniani et al. 2019; Yang et al. 2019a; Li et al. 2020b, and references therein). Average CO SLEDs of local AGN and starburst galaxies from the literature (Mashian et al. 2015; Rosenberg et al. 2015) are reported by the dashed orange and dotted blue lines, respectively. Shaded areas are the confidence limits (computed as the 16th and 85th percentile of the distribution, respectively). Violet squares are the Milky Way Galactic center CO-normalized ladders from Fixsen et al. (1999). Average CO SLEDs of starburst galaxies at z ∼ 1 − 1.6 and z ∼ 2 − 2.6 from ASPECS are shown as green and red lines, respectively. The dashed gray line is the expected thermalized CO SLED in the Rayleigh-Jeans limit.