In the Agilent Help section, the bandwidths are listed that correspond to each combination of memory length and time base.
For example, if you have selected an 11-s pretrigger delay, single-channel operation, and a 200-[micro]s/div time base, the sampling rate will be 500 kS/s for any memory length greater than 2 kwords.
Like conventional analog A/B operation, the B time base is constrained to be equal to or faster than the A time base.
You may think of the A intensified by B display mode as having to do with separate time bases and alternate sweeps, but it doesn't.
The first thing you may notice is that the time bases are arranged in a 1-2-4 sequence rather than the traditional 1-2-5 sequence.