You are not both inputs, therefore you only pay for your portion of the transaction.

You are wrong. If what you claim were actually implemented, it would create an attack vector where the sender creates a payjoin transaction at a high fee rate and burns the receiver's input.

Dude, I'm not going to argue with you suffice it to say, that's not how it works. This is not an inclient function this is a coordinated out-of-band operation. You don't just get to "set your fees." Go read this:

How it Works | Payjoin

Learn the technical details of the payjoin protocol

I just explained how your design lets a payjoin sender could drain the receiver's wallet. Do some research before spreading misinformation: "In exchange for the privacy benefit, the sender has to pay more fees than a normal transaction. It is a con for the sender, but a pro for the receiver, since the receiver does not have to consolidate its coin later." - @nopara73 (coinventor of payjoin) https://nopara73.medium.com/pay-to-endpoint-56eb05d3cac6