Not sure as I'm not familiair with sslh (I assume you mean GitHub - yrutschle/sslh: Applicative Protocol Multiplexer (e.g. share SSH and HTTPS on the same port) by that, right?) Its README doesn't contain much info unfortunately.
Although.. If I read it correctly.. sslh is a transparent proxy and does not function as a TLS endpoint at all.. That would mean your webserver behind sslh would terminate the TLS connection and you could run certbot just at your webserver behind the proxy. No need for multiple certificates.