It's always hilarious to me to hear words like "I assume" and all manner of carelessness when discussing 240v power. The J connection uses PWM signaling in both directions to communicate both the demand and the supply ability, and agree on a setting. There's more than just some simple resistance. You certainly can't have both bikes "talking" to the EVSE.
The connector is not powered until the above finishes. So you don't have arcs when you plug in, people can't be electrocuted, etc. In a simple cobbled-together splitter, one side would stay live after negotiation, but can't negotiate at all itself.
Applying input power to a J plug should NOT initiate charging without the communication. I haven't tried it yet, I should. But if the bikes meet standards, one will not charge.
I recently learned that one of my EVSEs won't stay on its 16a setting and reverts to the full 24a under certain circumstances. So there are even those quirks. Also although I successfully rigged my generator to work with an EV, it won't charge the Zero. That tries to take full power instantly, the generator can't speed up fast enough, and it stops. Repeat. The Rivian is fine because it ramps up power. So I need a switchable load if I want to do it (which I don't, it was a test).