Ok, looks like the CSS 404s was not “fixed” by disabling SSL for one of the sites - it still periodically stops serving on either/both of the sites. A restart of the web container for either instance will temporarily reinstate the CSS. Seems to be a problem of the CSS aggregation or static file serving… I’ve posted about it here: https://meta.discourse.org/t/intermittent-stylesheet-errors-on-heroku-installation/26261/12
I should note that I’m not aware of this problem (of 404s for CSS) arising prior to the introduction of the second discourse instance.