There is env -S that supports multiple arguments. This was always an extension available in BSD I think, and it is available now in recent versions of GNU's env.
GNU Coreutils env supports the -S option as of v8.30. Ubuntu 18.04 LTS appears to be on v8.28, but 19.04 supports it. (https://stackoverflow.com/q/4303128)
(Also, I didn't think the shebang was specified by POSIX at all? Am I wrong?)
(Also, I didn't think the shebang was specified by POSIX at all? Am I wrong?)