Heya I've got a secondhand oscilloscope that half works. The second trace works fine, but the other input doesn't ever display anything. I'd assume if it was something other than the CRT it would still display a flat line instead of just nothing, which means either part of the tube isn't getting power or part of the tube is just busted. If yall got any suggestions I'm all ears, otherwise I'll just be investigating how 2-trace CRTs work.

Edit: from wikipedia:

dual-trace oscilloscopes, [...] using a single-beam CRT, multiplex the inputs, usually switching between them fast enough to display two traces apparently at once

Ok so it seems the problem is likely not the tube. Manual is here if you care.