Thanks for the second response. It looks like my original goal got lost. I have a camera library that writes files to the onboard SD card (or any other) and the PC can read those files when the SD card is inserted into a USB port. The goal is to time slice access to an SD card between the Teensy and PC via a secondary USB connection to the PC since it appears MTP functions are WIP.
My reasoning is that if I used an SD unit that was not the onboard one, I could use an external MUX IC to select between the Teensy and the PC having access. In this config, the original protocol usage would already be sufficient to get the job done, and I don't have to go into depth on protocols. Unless I'm missing something, this at a minimum should work.
From there, I figured perhaps Teensy's pins could be configured as pass-throughs to save the need for the extra MUX...and if that works, perhaps they can be configured to pass through to the onboard SD card as well...which saves another external component as well. I'd need logic converters, but its much neater. This would of course revolve around being able to blindly ferry USB traffic from pin to pin WITHOUT foreknowledge of protocols. It seems the core message here is that foreknowledge IS required. Just in case someone outsmarted how do to this, I figured I'd ask.