A dual trace CRO (cathode ray oscilloscope) consists of only a single electron gun...
But it produce two traces... how??

Any display picture is build up of many frames per second.
A televison picture has 50 to 60 frames per second, which are interlaced into 2 x 25 or 2 x 30 pictures per second , to increase picture quality.

An oscilloscope produces more or less frames per second, depending on the setting.
For dual trace there are two different display signals used in time-lapse mode.
The first full picture frame displays the first signal, and the second frame the second signal. The third frame again the first signal, etc.
The effect is as if there are two beams. Actually there is only one beam at any one moment, but with constantly swopping signals at sequencing display frames.

:)