Multiple different length Windows for high resolution realtime FFT
You can, and you don't need to zero pad, but a little overlapping does come into play.

Skipping the gnarly details and hitting this with a conceptual approach.  Applies to two or more "levels".

Start with a longer frame for your low tones.  The key is you need to smooth your signal fairly heavily.  Lots of ways to do this.  Then do a DFT (what a FFT really is) on the long interval.  The reason for the smoothing is to eliminate alias frequencies at this level.  The effects of your smoothing will be frequency dependent, so there are formulas for the different smoothing techniques.  Do the inverse of the FFT of the smoothed signal with attenuation compensation applied.  Subtract this from your original signal.  You have now documented and removed your lower frequencies.  Divide the interval into smaller chunks, and do DFTs as normal at the top level.

"Pasting" them together can also be done in different ways.  Easiest is just to display one spectogram above the other as "display bands".  You can also rescale the output from being linear in frequency to exponential if you want to put your results on an octave scale.