Can I connect multiple STM32 to single CAN transceiver?
Now, I want the &quot;secondary µCs&quot; on each board to &quot;just passively
listen&quot; and use individual ID filters to only see relevant data (each
free CPU cycle counts).

Yes, if you are doing this then there is no problem having multiple micro controllers with their respective CAN_TX and CAN_RX pins tied together. As you mentioned, you should ensure they are set to passive mode if possible (which the STM32 does).
Note that even if they didn't have a passive mode, it should still be okay to connect the lines together (though some series resistors on CAN_TX would be prudent) as each CAN controller will ACK every message or generate error frames at the same point anyway. Also note that most CAN controllers will ACK any valid frame they see, regardless of whether it is subsequently filtered or not. Filtering is a higher level operation whereas the ACK bit is there to provide basic connectivity checking for the transmitting node (and not a whole lot else in practice).