Two mail providers for the same domain
you cant have two different providers for the same exact domain, but you can have a different subdomain for some emails.
you could have the important email at google , for [email protected] , pointing the MX records of yourdomainname.tld to google MX servers and less important emails on another , cheaper provider for [email protected] , pointing the MX records of mail.yourdomainname.tld to the other, cheaper, provider.
this is probably your best way to achieve what you need.