Is it possibile to connect to hEXs router a cAP ac using a PoE Ethernet cable and connect another cAP ac to the previous cAP ac using another PoE Ethernet cable (thus using only one power supply to feed the hEXs and the 2 cAP ac)?

The hEXs router should then power two cascaded cAP ac:
Power-supply --> hEXs --> poe-eth-cable --> cAP ac --> poe-eth-cable --> cAP ac

The hEXs router is powered by the standard power supply that comes out of the box.

Any suggestion is more than welcome!

VS

No. PoE-out limit on hEX S is 500mA and if using "stock" power adapter (at 24V), that translates into 12W. Single cAP ac power consumption is rated at 12W (without attachments). Additionally, "stock" pwoer adapter is rated at 1.2A, at 24V this is 28.8W. hEX S own consumption is 6W, leaving 22.8W for powered devices (and that's less than 2x12W for two cAP ac devices).

However, if you replace power adapter with a 48V one, things start to seem possible: 500mA at 48V is 24W and that happens to be total rated power consumption of two cAP ac devices. Beware that using 48V power adapter doesn't give any headroom for losses in wires (depending on UTP cable lengths this might not be exactly negligible). So you should better find a 54V power adapter (rated at minimum 0.6A) ... I know it's not a very common voltage, but it would very likely give you possibility to run things without any problems.

Not what you asked for, but as practical fix, a MT power injector can deliver 2A.
I use it when connecting 2 Powerboxes with only one place with a power outlet. For long distances there are 2 GPER on the PoE line. PoE out of the Powerbox on its own is too low max current, for the 2nd Powerbox to distribute.