The logic of my custom CheckBox does not work like the material’s default CheckBox, what can I do to solve this?

Here I have a column with two Material’s Checkbox, the logic behind them is: as soon as one is checked, the other is automatically unchecked

Column(
            children: [
              SizedBox(
                width: screenSize.width * .38,
                height: screenSize.height * .07,
                child: CardBox(
                  shadowSize: 2,
                  customRadius: const [10, 10],
                  boxContent: Row(
                    children: [
                      Transform.scale(
                        scale: 1.3,
                        child: Checkbox(
                          value: bool1Selected,
                          activeColor: widget.color,
                          checkColor: widget.color,
                          onChanged: (newBool) {
                            setState(() {
                              bool2Selected= false;
                              bool1Selected= newBool;
                            });
                          },
                        ),
                      ),
                      const SizedBox(
                        width: 5,
                        height: 1,
                      ),
                      Text('Spain',
                          style: const TextStyle(
                            fontSize: 35,
                            ),
                          ))
                    ],
                  ),
                ),
              ),
              SizedBox(
                width: screenSize.width * .38,
                height: screenSize.height * .02,
              ),
              SizedBox(
                width: screenSize.width * .38,
                height: screenSize.height * .07,
                child: CardBox(
                  shadowSize: 2,
                  customRadius: const [10, 10],
                  boxContent: Row(
                    children: [
                      Transform.scale(
                        scale: 1.3,
                        child: Checkbox(
                          value: bool2Selected,
                          activeColor: widget.color,
                          checkColor: widget.color,
                          onChanged: (newBool) {
                            setState(() {
                              bool1Selected = false;
                              bool2Selected = newBool;
                            });
                          },
                        ),
                      ),
                      const SizedBox(
                        width: 5,
                        height: 1,
                      ),
                      Text(
                        'EUA',
                        style: const TextStyle(
                          fontSize: 35,
                          
                          ),
                        ),
                      ),
                    ],
                  ),
                ),
              ),
            ],
          ),

With this code above, it is working, however I made a custom CheckBbox to better serve my purpose, but as soon as I click on one CheckBox the other is not unchecked. Follow the code below:

class _CustomCheckboxState extends State<CustomCheckbox> {
  Color borderColor = ThemeColors.cardHighlight;
  bool isChecked = false;

  @override
  void initState() {
    super.initState();
    isChecked = widget.isChecked;
  }

  @override
  Widget build(BuildContext context) {
    var screenSize = MediaQuery.of(context).size;

    return InkWell(
      child: Container(
        width: screenSize.width * .35,
        height: screenSize.height * .07,
        decoration: BoxDecoration(
          color: ThemeColors.cardHighlight,
          boxShadow: kElevationToShadow[3],
          borderRadius: const BorderRadius.all(Radius.circular(10)),
          border: Border.all(
            width: 6,
            strokeAlign: BorderSide.strokeAlignInside,
            color: isChecked ? borderColor = widget.borderColor : borderColor = ThemeColors.cardHighlight,
          ),
        ),
        child: Center(
          child: Text(
            widget.text,
            style: TextStyle(fontSize: 35),
          ),
        ),
      ),
      onTap: () {
        setState(
          () {
            isChecked = !isChecked;
            widget.onChange(isChecked);
          },
        );
      },
    );
  }
}

I tried to analyze the Material CheckBox source code, but a lot of things there like state management I’m still trying to learn in flutter, could anyone please tell me how to fix this problem?