Recording processed and direct (dry) guitar simultaneously in GarageBand

Here's a puzzle. A friend is trying to record guitar parts to send to me for review and collaboration. Using GarageBand (GB), we want a two-track session, with both tracks sharing the same mono audio input, in this case the mic/line input on his Mac.

GB does not permit arming both tracks to record if the inputs are the same. I've confirmed that two different physical inputs will enable arming both, but he currently does not have an audio interface to provide these.

While Logic Pro enables AUX send to mult the tracks, GB unfortunately does not.

In a different audio app, I've confirmed that Apple's AUNetSend and AUNetReceive plugins can do this, but Apple enables only AUNetSend and not AUNetReceive in GB. In other words, they have apparently made it impossible to record wet and dry signals in parallel using GB and the Mac audio input.

Has anyone found a solution for this? A simple pair of 3rd-party AU plugins to pass audio between tracks would be great, preferably freeware. If nothing like this exists, we may simply pass the GB project file back and forth.

Thanks in advance for your suggestions.