Interesting question, Bhante.
Current neurophysiology might shed some light on this.
Several studies, such as Science | AAAS, indicate there is in fact simultaneious (if less efficient) processing in the brain when processing more than one task, which may include parallel processing of multiple feelings.
So multiple feelings or thought-tasks are present, processed, and “known” pre-consciously one-at-a-time with respect to each process, as conditions arising in the conditional aspect of the body called “brain.”
But, as we know, the mind is also a “seamstress” rapidly, and usually pre-consciously, weaving together many pre-conscious processes into coherent feelings or sensations that appear to be simultaneous.
The point is, perhaps both sides have a point in terms of the suttas:
What subjectively appears as “back pain at the same time as bliss”, thanks to the weaving together of waking conscious is, in fact, an illusion because of the rapid toggling back-and-forth of the two processes, which are really experienced only one at a time.
There’s only one frame at a time during a movie and if our minds processed visual stimuli more rapidly we’d see each one, one at a time. But the toggling and combining of the brain and mind see it all at once. In this sense, multiple feelings are subjectively experienced simultaneously. But, in fact, they are not so.
Assuming the Buddha’s depth of passana allowed him to see the “only one at a time, frame by frame” aspect of feelings, along the lines of the study cited above, the weaving together of conscious experience that has them subjectively appear to be simultaneous would of course also be seen into.
Usual wakeful consciousness → “simultaneous feelings”; ultra-mindfulness → only ever one at a time, which the Buddha taught as the deep reality of feelings.
Perhaps in this context the teaching was offered as a skillful way to teach people to settle down and focus their minds rather than swinging from one feeling to the next, especially as the mind can be distracted by focusing on different feelings that appear to be simultaneous in consciousness, competing for attention. But can only theorize about this…