A and B are triggers, also called A sweep and B sweep. The scope is dual-timebase, which means that it can use two different horizontal sweep rates for the two different trigger events. This is a "zoom" function that allows you to magnify (horizontally) one particular part of the waveform. This is not the same as CH1 and CH2, you cannot have different timebases for the channels. The B-sweep cannot be slower than the A-sweep, so if you turn the B-sweep timebase knob counterclockwise, it should eventually catch the A-sweep knob and turn that as well. If it hasn't done that, then adjusting the B-sweep timebases should have no effect on your A-sweep appearance. The picture you posted shows the scope configured to only show the A-sweep but I can't read the B-sweep timebase setting so I don't know if this is an issue or not.