ChB is only relative to A so I don't find a way to measure frequency with it.
Regards
You can measure Frequency and Pulse on Ch B.
It's in the special menu, function 17 (FREQ B) and 20 (PULSE B). <SCALE>, <SMOOTH>, <1>, <7> and <ENTER> press is needed.
Description on page 3-21, 5335A_90021 manual.
Or send FN17 or FN20 over GPIB.
Switches cannot be changed so easily, because <ENTER> is version w/o LED, whereas VOLT key has LED.
Also, if you want to use the MATH functions including numerical inputs, mostly all keys are needed, like EEX on the VOLT key.
These old switches often feel as if they are blocked, when pressed, or the spring force is very weak.
In the first case, they might also need a harder press to make contact.
Frank