> We need a way to set multiple SSL certificates with overlapping duration.

Both Apache (SSLCertificateFile) and nginx (ssl_certificate) allow for multiple files, though they cannot be of the same algorithm: you can have one RSA, one ECC, etc, but not (say) an ECC and another ECC. (This may be a limitation of OpenSSL.)

So if the RSA expires on Feb 1, you can have the ECC expire on Feb 14 or Mar 1.