Is it possible using two 3.3v LDOs from one SMPS AC-DC 5V to power 2 MCUs?
Yes, it will work, as long as both devices share the same ground I don't see any reason why there should arise a problem. But I don't see the any reason to do it this way either.
It will be no problem to connect both MCUs to the same regulator. Just make sure to have good filtering for the analog supply of the STM. If you check the STM documents they explain very detailed what should be done to achieve clean input voltages for analog supply and reference: Put an RC-Filter in front of the input of the analog supply or even use a ferrite bead. This will attenuate switching noise from the MCUs and the SMPS.


  ● The VREF+ pin can be connected to the VDDA external power supply. If
  a separate, external reference voltage is applied on VREF+, a 100 nF
  and a 1 μF capacitors must be connected on this pin. In all cases,
  VREF+ must be kept between 2.4 V and VDDA.
  
  ● Additional precautions can be taken to filter analog noise:
    - VDDA can be connected to VDD through a ferrite bead.
    - The VREF+ pin can be connected to VDDA through a resistor (typ. 47 Ω).