The only difference between singing and humming is really where the air is going. Since you're directing the air through your mouth to form a whistle, this can't be humming, you're actually singing and whistling at the same time.
When I taught myself how to do this, I did notice that the two pitches were moving together at first, but I'll bet if you worked on it in the right way you could learn to control the distinct muscle groups individually. This is actually rather similar to multiphonics on a brass instrument, where the lip buzz and the sung tone must be controlled individually in many cases.
There is probably some innate neural reason why we have trouble with this; whatever it is, it is clear that 99% of human-controlled musical tones are single line, so we have a very strong neural link between a single pitch and a muscular setting. Audiating two pitches at a time and then controlling different muscle groups for each is a pretty tall order given that context, but it can be done.
Here are some things you can try:
- While singing or whistling alone, identify the minimum amount of musculature that you can move to effect a change in pitch. For me, I can change a pitch in my whistle by moving the tongue only, and can change sung pitch by only moving the throat.
- Become very familiar with those muscular movements
- When singing and whistling simultaneously, don't think about the notes. Think instead of individual muscle groups.
Given that, try singing and whistling simultaneously. Once you've got it steady, try moving the tongue back and forth. If you're only thinking about the muscle and not about the note, you should hear a change in pitch of the whistle. Changing sung note while sustaining a whistled one is the same thing with the opposite muscle group. I'd recommend starting with the tongue because it's a very easy muscle to localize your brain's control over.
After you've got that down, then it's just a matter of refining control and linking each individual muscle group back to your musical mind. Good luck!