Set the scope mainframe to Vert Mode set on CHOP and Trig Source set to VERT MODE as seen here on the upper right.
On the modules, set the 7A18 (sorry one of my knobs is broken (middle module) but its set the same as the one on the left.
Edit: I see both of your DISPLAY MODE knobs on both 7A18s are starting to crack as well, maybe you can try to repair them before they go away.Set the Trigger Source to MODE on both dual trace amplifiers, and set the Display Source to CHOP on both as well. I did changed my input coupling to GND as you had but I would leave them on AC to test.
Then I adjusted the 4 channel positions just to separate the traces.
On my Time base module (mine is a 7B53A) I have the Mode set to AUTO, the Coupling set to AC and the Source set to INT, all of the rest should be like yours. I did change mine to 100nS but I don't think that would matter. Also mine doesn't have a contrast adjustment so I have no idea what that does on your 7B92
Also on the mainframe the inner intensity knob controls the extra information (READOUT) like the 100mV or 100nS in the picture, the outer one is for the trace intensity.
I would also check if your GRATICULE ILLUMINATION is affected as well.
Here is a picture of my setup that shows all four traces: