Previous post is absolutely correct. Do NOT try to connect the two antennas together. It cannot do anything but make things worse.
Theoretically the diversity antenna, with a HU that is equipped for it, would help reduce "picket-fencing" and similar FM ills. It's not going to do much for AM because, well, AM doesn't have those sorts of problems; the wavelength is far too long for that.
In practice I actually think the diversity antenna in our Supras cannot do much anyway, as it is too close to the whip. However it's a distinctly different shape and orientation, so... maybe. Maybe the idea is that it picks up horizontally polarized FM signals while the whip does better at vertical.
At one time I actually had an aftermarket HU, a high-end Sony, that had the two antenna inputs. But I never did find an adapter for the Supra's plug. I ended up just replacing the plug with a standard Motorola plug. It really did work as an antenna (verified by disconnecting the main and plugging in a 75 ohm terminator in its place) but performance was much worse than with the whip.
Incidentally if you try to measure continuity between the connector to the glass and the center conductor on the plug, you will find none. That's ok, there's a small capacitor in series. Passes RF, blocks DC.