Listen on both HTTP and HTTPS

Is it possible to use both HTTP and HTTPS at the same time when creating API? 

A single API definition will only listen on a single protocol so you will need to create 2 API definitions.
 
Please note that a listening port can only serve a single protocol so it will be necessary to enable two ports to serve both HTTP and HTTPS.
 
There is a discussion of port whitelisting at https://tyk.io/docs/key-concepts/tcp-proxy/ which will provide details on how to set up the gateway config to support this.