Is it a golden rule to not titrate a weak acid against a weak base?

You just answered your own question.
Weak acids and weak bases are slightly ionized.
The end point will not be sharp at or near the equivalence point.
A 'single weak acid' must be titrated with a strong base like NaOH and
your primary standard for NaOH is KHP.
A 'single weak base' must be titrated with a strong acid like HCl and your primary standard is Na2CO3. If you use phenolphthalein as an indicator to approximate the first inflection point then use Bromcresol green as an indicator to detect the final end point (second inflection). Methyl orange indicator alone will not give you an accurate end point.  The methyl orange indicator must be modified by adding some kind of dye.
It follows that if you were using Na2CO3 as your primary standard your titrant or titrating agent is a strong acid, say HCl. Why would you use HCl to find the acid content in your sample? Are you back titrating or spiking your sample by adding a strong base to your sample?
I believe this is not a good practice. It is a common practice to employ back titration in Iodimetry or Iodometry (adding excess I2 and back titrating with Na2S2O3).
Good Luck !!!!