In bug 1595031 we implemented support for wasm functions returning multiple values to JS, and JS functions returning multiple values to wasm. However calls are only supported via the InterpEntry / InterpExit; the JIT ABI isn't supported. See bug 1595031 for a discussion of the issues.
Optimization item, bumping to P3.
Not urgent until multi-value ABI becomes a thing for wasm producers, if then. Also we await JS tuples. Also we await a better system for generating stubs code, including rooting support.