Is it possible to built a symmetric dc to dc converter (e.g. +/12Vdc) using only one mc34063. If it's not possible how can i get the middle point of the measurement (+12V com -12V) by using two of them. Any ideas?
That IC has an App Note for its Buck-Boost configuration adapted to dual output with a transformer with twice the inductance per leg. Since it is switched high side, you can connect the floating secondary to Gnd. The current load ought to be similar since you can only regulate one output with one regulator. https://www.st.com/internet/com/TEC...AL_LITERATURE/APPLICATION_NOTE/CD00043744.pdf p28
The secondary output may be ~10% lower depending on mutual inductance or coupling. I suggest you wind your own torroid with bi-filar windings and experiment with secondary going into a diode cap of similar size as primary side.
To get symmetrical (tracking) dual voltage output, the primary output must be connected as "inverting boost" (flyback) converter. The second winding will generate the positive output voltage.