line profiles to be very similar, suggesting that they originate from the same source-plane regions. However, the limited spatial resolution of these observations prevented a robust source-plane reconstruction of the CO/\[\] emission; the *differential magnification* bias (e.g.,@Serjeant2012) therefore could not be eliminated. In addition, several individual sources at $z>4.5$ have been studied in both \[\] and low-$J$ CO emission (e.g.,@Walter2009 [@Huynh2014; @Cicone2015]), though these tend to be extreme sources in terms of FIR brightness and AGN activity.

Finally, the \[\] line has been proposed as an alternative to CO emission as a molecular gas tracer (e.g.,@Zanella2018). However, in SMGs, the spatial extent of CO emission has been shown to vary strongly with $J_\mathrm{upp}$ (e.g.,@Ivison2011 [@Riechers2011]). If \[\] emission traces only a subset of the molecular gas reservoir, \[\]-based mass estimates might be severely biased.

In this paper, we explore a new regime in resolved, multi-tracer studies by investigating resolved \[\], FIR continuum and CO(3–2) emission on kpc-scales in two (unlensed) $z\sim3$ sources. This allows us to address the following questions:

-   How does the resolved \[\]/FIR ratio at $z\sim$3 compare to that seen in local and high-redshift star-forming galaxies?

-   What physical mechanism drives the \[\]/FIR deficit in SMGs?

-   