Can two usb gaming controllers be wired together to act as one?
This would have to be handled by software. Either the OS, a driver or middleman software, or the game. Many games allow you to arbitrarily assign one or multiple buttons to an action, regardless from which source. Some middleman programs can intercept and override the controllers to turn joystick events into keyboard events.

Some controllers have a usb keyboard mode that would work too.

But a hardwired usb physical method? Short of replacing it's hardware so the physical buttons are wired together, no.