Common namespace in multiple translation units

Ok., let it be:
allDisplays.h

// display factory class
#include <genieArduino.h>
Genie uLCD90DCT;
// my classes
#include "InvDisplay.h"
InvDisplay* invDisplays = new InvDisplay[6];
#include "smallDisplay.h"
smallDisplay display;

allDisplays.ino

#include "allDisplays.h"
void setup() {
  Serial1.begin(9600);
  uLCD90DCT.Begin(Serial1);
  uLCD90DCT.AttachEventHandler(displayEventHandler);  // Attach the user function Event Handler for processing events
  for (uint8_t i = 0; i < 6; i++) {
    invDisplays[i].init(i);
  }
}
void displayEventHandler() {
  genieFrame Event;
  uLCD90DCT.DequeueEvent(&Event);
  // later do something, if a button is pressed
}
void loop() {
  uLCD90DCT.DoEvents();  // This calls the library each loop to process the queued responses from the display
}

my classes
smallDisplay.h

#ifndef smallDisplay_h
#define smallDisplay_h

#include <Arduino.h>
#include <String.h>
#include <genieArduino.h>

const uint8_t a1aktI_LeddigitsID = 6;

class smallDisplay {
public:
  smallDisplay();
  void setA1aktI(uint16_t value);
private:
  void _setGENIE_OBJ_LED_DIGITS(uint8_t _LeddigitsID, uint16_t value);
};
#endif

smallDisplay.cpp

#include "smallDisplay.h"

smallDisplay::smallDisplay() {
}

void smallDisplay::setA1aktI(uint16_t value) {
  _setGENIE_OBJ_LED_DIGITS(a1aktI_LeddigitsID, value);
}

void smallDisplay::_setGENIE_OBJ_LED_DIGITS(uint8_t _LeddigitsID, uint16_t value) {
  uLCD90DCT.WriteObject(GENIE_OBJ_LED_DIGITS, _LeddigitsID, value);
}

InvDisplay.h

#ifndef InvDisplay_h
#define InvDisplay_h

#include <Arduino.h>
#include <String.h>
#include <genieArduino.h>
const uint8_t invSetU_LeddigitsID_arr[] = { 2, 8, 9, 10, 11, 12 };

class InvDisplay {
public:
  InvDisplay();
  void init(uint8_t arrIdx);
  void setInvSetU(uint16_t value);

private:
  void _setGENIE_OBJ_LED_DIGITS(uint8_t _LeddigitsID, uint16_t value);
  uint8_t _invSetU_LeddigitsID;
};
#endif

InvDisplay.cpp

#include "InvDisplay.h"

InvDisplay::InvDisplay() {
}

void InvDisplay::init(uint8_t arrIdx) {
  _invSetU_LeddigitsID = invSetU_LeddigitsID_arr[arrIdx];
}

void InvDisplay::setInvSetU(uint16_t value) {
  _setGENIE_OBJ_LED_DIGITS(_invSetU_LeddigitsID, value);
}

void InvDisplay::_setGENIE_OBJ_LED_DIGITS(uint8_t _LeddigitsID, uint16_t value) {
  uLCD90DCT.WriteObject(GENIE_OBJ_LED_DIGITS, _LeddigitsID, value);
}

This is the complete test code.