You can't use both. In Primus you can select only one transformer, listed above by V1, but if something doesn't work, you can just switch it with a single line of code. No rewrites, no need to learn a new API.

I know you can't use both, but doesn't engine.io already support websockets?

Yes it does, but if you are building something that works perfectly only using websockets (e.g. a command-line utility), why would you use engine.io?

You can tell engine.io to only use websockets, but what's the point if you can use websockets directly and have the same API?