​Right now we just need a few modifications to Async to make this work (Lwt using Async backend or vice versa).​

It's relatively easy to get into a state with the limitation that you couldn't use the same file descriptor with Lwt and Async, which seems acceptable. For stdout/stderr it'd be fine as long as you don't print at the same time, in which case the output might be interleaved. But with a bit more work on Lwt and Async this could be sorted